diff --git a/LibraryApp.Domain/BookCountReport.cs b/LibraryApp.Domain/BookCountReport.cs
new file mode 100644
index 0000000..328e112
--- /dev/null
+++ b/LibraryApp.Domain/BookCountReport.cs
@@ -0,0 +1,30 @@
+using LibraryApp.Domain.Entities;
+
+namespace LibraryApp.Domain;
+
+public class BookCountReport
+{
+    public ShelfId ShelfId { get; }
+    public DateTime CountedAt { get; }
+    public IReadOnlyList<BookInstance> MissingInstances { get; }
+    public int TotalExpected { get; }
+    public int TotalFound { get; }
+
+    public BookCountReport(
+        ShelfId shelfId,
+        DateTime countedAt,
+        IReadOnlyList<BookInstance> allInstancesOnShelf,
+        IReadOnlyList<BookInstanceBarcode> scannedBarcodes)
+    {
+        ShelfId = shelfId;
+        CountedAt = countedAt;
+        TotalExpected = allInstancesOnShelf.Count;
+
+        var scannedSet = scannedBarcodes.Select(b => b.Barcode).ToHashSet();
+        MissingInstances = allInstancesOnShelf
+            .Where(i => !scannedSet.Contains(i.Barcode.Barcode))
+            .ToList();
+
+        TotalFound = TotalExpected - MissingInstances.Count;
+    }
+}
diff --git a/LibraryApp.Domain/Class1.cs b/LibraryApp.Domain/Class1.cs
deleted file mode 100644
index 3b7e276..0000000
--- a/LibraryApp.Domain/Class1.cs
+++ /dev/null
@@ -1,6 +0,0 @@
-namespace LibraryApp.Domain;
-
-public class Class1
-{
-
-}
\ No newline at end of file
diff --git a/LibraryApp.Domain/DbId.cs b/LibraryApp.Domain/DbId.cs
new file mode 100644
index 0000000..d31c320
--- /dev/null
+++ b/LibraryApp.Domain/DbId.cs
@@ -0,0 +1,13 @@
+namespace LibraryApp.Domain;
+
+public record DbId
+{
+    public string Id { get; init; }
+    public bool BlankId => string.IsNullOrWhiteSpace(Id);
+    public DbId(string id)
+    {
+        if(id is null)
+            throw new ArgumentNullException(nameof(id));
+        Id = id;
+    }
+}
diff --git a/LibraryApp.Domain/Entities/Book.cs b/LibraryApp.Domain/Entities/Book.cs
new file mode 100644
index 0000000..6a54e4c
--- /dev/null
+++ b/LibraryApp.Domain/Entities/Book.cs
@@ -0,0 +1,65 @@
+namespace LibraryApp.Domain.Entities;
+
+public class Book
+{
+    public BookId Id { get; private set; }
+    public BookTitle Title { get; private set; }
+    public BookAuthor Author { get; private set; }
+
+    public Book(BookTitle title, BookAuthor author) 
+        : this(new BookId(string.Empty), title, author)
+    { }
+
+    public Book(BookId id, BookTitle title, BookAuthor author)
+    {
+        Id = id;
+        Title = title;
+        Author = author;
+    }
+
+    public Book UpdateTitle(string newTitle)
+    {
+        Title = new (newTitle);
+        return this;
+    }
+    public Book UpdateAuthor(string newAuthor)
+    {
+        Author = new (newAuthor);
+        return this;
+    }
+}
+
+public record BookId : DbId
+{
+    public BookId(string id) : base(id) {}
+}
+
+public record BookTitle
+{
+    public string Title { get; init; }
+
+    public BookTitle(string title)
+    {
+        if(string.IsNullOrWhiteSpace(title))
+        {
+            throw new ArgumentException("Title cannot be empty.");
+        }
+        Title = title;
+    }
+}
+
+public record BookAuthor
+{
+    public string Author { get; init; }
+
+    public BookAuthor(string author)
+    {
+        if(string.IsNullOrWhiteSpace(author))
+        {
+            throw new ArgumentException("Author cannot be empty.");
+        }
+        Author = author;
+    }
+}
+
+
diff --git a/LibraryApp.Domain/Entities/BookInstance.cs b/LibraryApp.Domain/Entities/BookInstance.cs
new file mode 100644
index 0000000..799a747
--- /dev/null
+++ b/LibraryApp.Domain/Entities/BookInstance.cs
@@ -0,0 +1,99 @@
+namespace LibraryApp.Domain.Entities;
+
+public enum BookInstanceStatus
+{
+    OnShelf,
+    Lent,
+    PutAside,
+    Lost
+}
+
+public class BookInstance
+{
+    public BookInstanceId Id { get; private set; }
+    public BookId BookId { get; private set; }
+    public ShelfId ShelfId { get; private set; }
+    public BookInstanceBarcode Barcode { get; private set; }
+    public BookInstanceStatus Status { get; private set; }
+
+    public BookInstance(BookId bookId, ShelfId shelfId, BookInstanceBarcode barcode)
+        : this(new BookInstanceId(string.Empty), bookId, shelfId, barcode, BookInstanceStatus.OnShelf)
+    { }
+
+    public BookInstance(BookInstanceId id, BookId bookId, ShelfId shelfId, BookInstanceBarcode barcode, BookInstanceStatus status)
+    {
+        Id = id;
+        BookId = bookId;
+        ShelfId = shelfId;
+        Barcode = barcode;
+        Status = status;
+    }
+
+    public BookInstance MarkAsLent()
+    {
+        if (Status != BookInstanceStatus.OnShelf)
+            throw new InvalidOperationException($"Cannot lend a book that is not on the shelf. Current status: {Status}.");
+        Status = BookInstanceStatus.Lent;
+        return this;
+    }
+
+    public BookInstance ReturnToShelf()
+    {
+        if (Status != BookInstanceStatus.Lent)
+            throw new InvalidOperationException($"Cannot return a book that is not lent out. Current status: {Status}.");
+        Status = BookInstanceStatus.OnShelf;
+        return this;
+    }
+
+    public BookInstance MarkAsPutAside()
+    {
+        if (Status != BookInstanceStatus.OnShelf)
+            throw new InvalidOperationException($"Cannot put aside a book that is not on the shelf. Current status: {Status}.");
+        Status = BookInstanceStatus.PutAside;
+        return this;
+    }
+
+    public BookInstance PlaceOnShelf(ShelfId shelfId)
+    {
+        if (Status != BookInstanceStatus.PutAside)
+            throw new InvalidOperationException($"Cannot place on shelf a book that is not put aside. Current status: {Status}.");
+        ShelfId = shelfId;
+        Status = BookInstanceStatus.OnShelf;
+        return this;
+    }
+
+    public BookInstance MarkAsLost()
+    {
+        if (Status != BookInstanceStatus.OnShelf)
+            throw new InvalidOperationException($"Cannot mark as lost a book that is not on the shelf. Current status: {Status}.");
+        Status = BookInstanceStatus.Lost;
+        return this;
+    }
+
+    public BookInstance UnmarkAsLost()
+    {
+        if (Status != BookInstanceStatus.Lost)
+            throw new InvalidOperationException($"Cannot unmark as lost a book that is not marked as lost. Current status: {Status}.");
+        Status = BookInstanceStatus.OnShelf;
+        return this;
+    }
+}
+
+public record BookInstanceId : DbId
+{
+    public BookInstanceId(string id) : base(id) {}
+}
+
+public record BookInstanceBarcode
+{
+    public string Barcode { get; init; }
+
+    public BookInstanceBarcode(string barcode)
+    {
+        if(string.IsNullOrWhiteSpace(barcode))
+        {
+            throw new ArgumentException("Barcode cannot be empty.");
+        }
+        Barcode = barcode;
+    }
+}
diff --git a/LibraryApp.Domain/Entities/LendRecord.cs b/LibraryApp.Domain/Entities/LendRecord.cs
new file mode 100644
index 0000000..ddbd8dc
--- /dev/null
+++ b/LibraryApp.Domain/Entities/LendRecord.cs
@@ -0,0 +1,72 @@
+namespace LibraryApp.Domain.Entities;
+
+public class LendRecord
+{
+    public LendRecordId Id { get; private set; }
+    public BookInstanceId BookInstanceId { get; private set; }
+    public MemberId MemberId { get; private set; }
+    public LendCode Code { get; private set; }
+    public DateTime LendDate { get; private set; }
+    public DateTime? ReturnDate { get; private set; }
+
+    public LendRecord(BookInstanceId bookInstanceId, MemberId memberId, LendCode code)
+        : this(new LendRecordId(string.Empty), bookInstanceId, memberId, code, DateTime.UtcNow, null)
+    { }
+
+    public LendRecord(LendRecordId id, BookInstanceId bookInstanceId, MemberId memberId, LendCode code, DateTime lendDate, DateTime? returnDate)
+    {
+        Id = id;
+        BookInstanceId = bookInstanceId;
+        MemberId = memberId;
+        Code = code;
+        LendDate = lendDate;
+        ReturnDate = returnDate;
+    }
+
+    public LendRecord MarkAsReturned()
+    {
+        if(ReturnDate != null)
+            throw new InvalidOperationException("This lend record is already marked as returned.");
+        ReturnDate = DateTime.UtcNow;
+        return this;
+    }
+
+    public LendRecord UpdateLendDate(DateTime newLendDate)
+    {
+        if(ReturnDate != null)
+            throw new InvalidOperationException("Cannot update lend date for a record that is already marked as returned.");
+        LendDate = newLendDate;
+        return this;
+    }
+
+    public LendRecord UnmarkAsReturned()
+    {
+        if(ReturnDate == null)
+            throw new InvalidOperationException("This lend record is not marked as returned.");
+        ReturnDate = null;
+        return this;
+    }
+}
+
+public record LendRecordId : DbId
+{
+    public LendRecordId(string id) : base(id) {}
+}
+
+/// <summary>
+/// A short, human-friendly code written on the physical lend slip alongside the member's name.
+/// Must be unique per lend record and no longer than 8 characters.
+/// </summary>
+public record LendCode
+{
+    public string Code { get; init; }
+
+    public LendCode(string code)
+    {
+        if (string.IsNullOrWhiteSpace(code))
+            throw new ArgumentException("Lend code cannot be empty.");
+        if (code.Length > 8)
+            throw new ArgumentException("Lend code cannot exceed 8 characters.");
+        Code = code.ToUpperInvariant();
+    }
+}
\ No newline at end of file
diff --git a/LibraryApp.Domain/Entities/Member.cs b/LibraryApp.Domain/Entities/Member.cs
new file mode 100644
index 0000000..6ce55b0
--- /dev/null
+++ b/LibraryApp.Domain/Entities/Member.cs
@@ -0,0 +1,119 @@
+namespace LibraryApp.Domain.Entities;
+
+public class Member
+{
+    public MemberId Id { get; private set; }
+    public MemberName Name { get; private set; }
+    public List<MemberEmail> Emails { get; private set; } = new();
+    public List<MemberPhone> Phones { get; private set; } = new();
+    public int PrimaryEmailIndex { get; private set; } = 0;
+    public int PrimaryPhoneIndex { get; private set; } = 0;
+
+    public Member(MemberName name, MemberEmail email, MemberPhone phone) 
+        : this(new MemberId(string.Empty), name, email, phone)
+    { }
+    public Member(MemberId id, MemberName name, MemberEmail email, MemberPhone phone)
+    {
+        
+        Id = id;
+        Name = name;
+        Emails.Add(email);
+        Phones.Add(phone);
+    }
+
+    public Member UpdateName(string newName)
+    {
+        Name = new (newName);
+        return this;
+    }
+    public Member AddEmail(string email)
+    {
+        Emails.Add(new MemberEmail(email));
+        return this;
+    }
+    public Member AddPhone(string phone)
+    {
+        Phones.Add(new MemberPhone(phone));
+        return this;
+    }
+    public Member SetPrimaryEmail(MemberEmail email)
+    {
+        var index = Emails.FindIndex(e => e.Email == email.Email);
+        if(index == -1)
+            throw new ArgumentException("Email not found in member's email list.", nameof(email));
+        PrimaryEmailIndex = index;
+        return this;
+    }
+    public Member SetPrimaryPhone(MemberPhone phone)
+    {
+        var index = Phones.FindIndex(p => p.Phone == phone.Phone);
+        if(index == -1)
+            throw new ArgumentException("Phone not found in member's phone list.", nameof(phone));
+        PrimaryPhoneIndex = index;
+        return this;
+    }
+}
+
+public record MemberId : DbId
+{
+    public MemberId(string id) : base(id) {}
+}
+
+public record MemberEmail
+{
+    public string Email { get; init; }
+
+    public MemberEmail(string email)
+    {
+        if(string.IsNullOrWhiteSpace(email))
+            throw new ArgumentException("Email cannot be empty.");
+
+        if(!email.Contains("@"))
+            throw new ArgumentException("Email must contain '@'.");
+
+        if(!email.Contains("."))
+            throw new ArgumentException("Email must contain '.'.");
+
+        if(email.StartsWith("@") || email.EndsWith("@"))
+            throw new ArgumentException("Email cannot start or end with '@'.");
+
+        var atCount = email.Count(c => c == '@');
+        if(atCount > 1)
+            throw new ArgumentException("Email cannot contain more than one '@'.");
+
+        var dotCount = email.Count(c => c == '.');
+        if(dotCount > 1)
+            throw new ArgumentException("Email cannot contain more than one '.'.");
+        
+        Email = email;
+    }
+}
+
+public record MemberName
+{
+    public string Name { get; init; }
+
+    public MemberName(string name)
+    {
+        if(string.IsNullOrWhiteSpace(name))
+        {
+            throw new ArgumentException("Member name cannot be empty.");
+        }
+        Name = name;
+    }
+}
+
+public record MemberPhone
+{
+    private const string VALID_PHONE_PATTERN = @"^\+?[1-9]\d{1,14}$";
+    public string Phone { get; init; }
+
+    public MemberPhone(string phone)
+    {
+        if(string.IsNullOrWhiteSpace(phone))
+            throw new ArgumentException("Phone number cannot be empty.");
+        if(!System.Text.RegularExpressions.Regex.IsMatch(phone, VALID_PHONE_PATTERN))
+            throw new ArgumentException("Phone number is not in a valid format.");
+        Phone = phone;
+    }
+}
diff --git a/LibraryApp.Domain/Entities/Shelf.cs b/LibraryApp.Domain/Entities/Shelf.cs
new file mode 100644
index 0000000..42518ad
--- /dev/null
+++ b/LibraryApp.Domain/Entities/Shelf.cs
@@ -0,0 +1,44 @@
+namespace LibraryApp.Domain.Entities;
+
+public class Shelf
+{
+    public ShelfId Id { get; private set; }
+    public ShelfName Name { get; private set; }
+    public DateTime? LastCountedAt { get; private set; }
+
+    public Shelf(ShelfName name)
+        : this(new ShelfId(string.Empty), name, null)
+    { }
+
+    public Shelf(ShelfId id, ShelfName name, DateTime? lastCountedAt)
+    {
+        Id = id;
+        Name = name;
+        LastCountedAt = lastCountedAt;
+    }
+
+    public Shelf RecordCount()
+    {
+        LastCountedAt = DateTime.UtcNow;
+        return this;
+    }
+}
+
+public record ShelfId : DbId
+{
+    public ShelfId(string id) : base(id) {}
+}
+
+public record ShelfName
+{
+    public string Name { get; init; }
+
+    public ShelfName(string name)
+    {
+        if(string.IsNullOrWhiteSpace(name))
+        {
+            throw new ArgumentException("Shelf name cannot be empty.");
+        }
+        Name = name;
+    }
+}
diff --git a/LibraryApp.Domain/Error.cs b/LibraryApp.Domain/Error.cs
new file mode 100644
index 0000000..5a01d10
--- /dev/null
+++ b/LibraryApp.Domain/Error.cs
@@ -0,0 +1,14 @@
+namespace LibraryApp.Domain;
+
+public record Error
+{
+    public string Message { get; init; }
+
+    public Error(string message)
+    {
+        Message = message;
+    }
+
+    public static implicit operator Error(string message) => new Error(message);
+    public static implicit operator string(Error error) => error.Message;
+}
\ No newline at end of file
diff --git a/LibraryApp.Domain/Repositories/IBooks.cs b/LibraryApp.Domain/Repositories/IBooks.cs
new file mode 100644
index 0000000..f1203e6
--- /dev/null
+++ b/LibraryApp.Domain/Repositories/IBooks.cs
@@ -0,0 +1,56 @@
+using LibraryApp.Domain.Entities;
+
+namespace LibraryApp.Domain.Repositories;
+
+public interface IBookRepository
+{
+    public Task<Result<Book>> GetAsync(BookId id);
+    public Task<Result<Book>> UpdateAsync(Book book);
+    public Task<Result<List<Book>>> GetAllAsync(int pageNumber, int pageSize);
+    public Task<Result<Book>> AddAsync(Book book);
+    public Task<Result> DeleteAsync(BookId id);
+}
+
+public interface IBookInstanceRepository
+{
+    public Task<Result<BookInstance>> GetByIdAsync(BookInstanceId id);
+    public Task<Result<BookInstance>> GetByBarcodeAsync(BookInstanceBarcode barcode);
+    public Task<Result<List<BookInstance>>> GetAllByShelfIdAsync(ShelfId shelfId);
+    public Task<Result<List<BookInstance>>> GetAllByStatusAsync(BookInstanceStatus status);
+    public Task<Result<BookInstance>> UpdateAsync(BookInstance bookInstance);
+    public Task<Result<List<BookInstance>>> GetAllAsync(int pageNumber, int pageSize);
+    public Task<Result<BookInstance>> AddAsync(BookInstance bookInstance);
+    public Task<Result> DeleteAsync(BookInstanceId id);
+}
+
+public interface IShelfRepository
+{
+    public Task<Result<Shelf>> GetAsync(ShelfId id);
+    public Task<Result<Shelf>> GetByBookInstanceBarcodeAsync(BookInstanceBarcode barcode);
+    public Task<Result<Shelf>> GetNextForCountAsync();
+    public Task<Result<Shelf>> UpdateAsync(Shelf shelf);
+    public Task<Result<List<Shelf>>> GetAllAsync(int pageNumber, int pageSize);
+    public Task<Result<Shelf>> AddAsync(Shelf shelf);
+    public Task<Result> DeleteAsync(ShelfId id);
+}
+
+public interface IMemberRepository
+{
+    public Task<Result<Member>> GetAsync(MemberId id);
+    public Task<Result<Member>> UpdateAsync(Member member);
+    public Task<Result<List<Member>>> GetAllAsync(int pageNumber, int pageSize);
+    public Task<Result<Member>> AddAsync(Member member);
+    public Task<Result> DeleteAsync(MemberId id);
+}
+
+public interface ILendRecordRepository
+{
+    public Task<Result<LendRecord>> GetAsync(LendRecordId id);
+    public Task<Result<List<LendRecord>>> GetAllByMemberIdAsync(MemberId memberId);
+    public Task<Result<LendRecord?>> GetActiveByBookInstanceIdAsync(BookInstanceId bookInstanceId);
+    public Task<bool> ExistsWithCodeAsync(LendCode code);
+    public Task<Result<LendRecord>> UpdateAsync(LendRecord lendRecord);
+    public Task<Result<List<LendRecord>>> GetAllAsync(int pageNumber, int pageSize);
+    public Task<Result<LendRecord>> AddAsync(LendRecord lendRecord);
+    public Task<Result> DeleteAsync(LendRecordId id);
+}
\ No newline at end of file
diff --git a/LibraryApp.Domain/Result.cs b/LibraryApp.Domain/Result.cs
new file mode 100644
index 0000000..0ddbead
--- /dev/null
+++ b/LibraryApp.Domain/Result.cs
@@ -0,0 +1,23 @@
+namespace LibraryApp.Domain;
+
+public record Result
+{
+    public required bool IsSuccess { get; init; }
+    public Error? ErrorMessage { get; init; }
+
+    private Result() { }
+
+    public static Result Success() => new Result { IsSuccess = true };
+    public static Result Failure(string errorMessage) => new Result { IsSuccess = false, ErrorMessage = errorMessage };
+}
+public record Result<T>
+{
+    public required bool IsSuccess { get; init; }
+    public T? Value { get; init; }
+    public Error? ErrorMessage { get; init; }
+
+    private Result() { }
+
+    public static Result<T> Success(T value) => new Result<T> { IsSuccess = true, Value = value };
+    public static Result<T> Failure(string errorMessage) => new Result<T> { IsSuccess = false, ErrorMessage = errorMessage };
+}
\ No newline at end of file
diff --git a/LibraryApp.Domain/Services/BookCountService.cs b/LibraryApp.Domain/Services/BookCountService.cs
new file mode 100644
index 0000000..fb1cdf0
--- /dev/null
+++ b/LibraryApp.Domain/Services/BookCountService.cs
@@ -0,0 +1,31 @@
+using LibraryApp.Domain;
+using LibraryApp.Domain.Entities;
+
+namespace LibraryApp.Domain.Services;
+
+public class BookCountService
+{
+    private readonly IBookCountReportPrinter _printer;
+
+    public BookCountService(IBookCountReportPrinter printer)
+    {
+        _printer = printer;
+    }
+
+    public async Task<Result<BookCountReport>> PerformCountAsync(
+        Shelf shelf,
+        IReadOnlyList<BookInstanceBarcode> scannedBarcodes,
+        IReadOnlyList<BookInstance> allInstancesOnShelf)
+    {
+        shelf.RecordCount();
+
+        var report = new BookCountReport(shelf.Id, DateTime.UtcNow, allInstancesOnShelf, scannedBarcodes);
+
+        foreach (var instance in report.MissingInstances)
+            instance.MarkAsLost();
+
+        await _printer.PrintAsync(report);
+
+        return Result<BookCountReport>.Success(report);
+    }
+}
diff --git a/LibraryApp.Domain/Services/IBookCountReportPrinter.cs b/LibraryApp.Domain/Services/IBookCountReportPrinter.cs
new file mode 100644
index 0000000..98a030f
--- /dev/null
+++ b/LibraryApp.Domain/Services/IBookCountReportPrinter.cs
@@ -0,0 +1,6 @@
+namespace LibraryApp.Domain.Services;
+
+public interface IBookCountReportPrinter
+{
+    Task PrintAsync(BookCountReport report);
+}
diff --git a/LibraryApp.Domain/Services/LendingService.cs b/LibraryApp.Domain/Services/LendingService.cs
new file mode 100644
index 0000000..44769cb
--- /dev/null
+++ b/LibraryApp.Domain/Services/LendingService.cs
@@ -0,0 +1,49 @@
+using LibraryApp.Domain.Entities;
+using LibraryApp.Domain.Repositories;
+
+namespace LibraryApp.Domain.Services;
+
+public class LendingService
+{
+    private readonly ILendRecordRepository _lendRecords;
+
+    public LendingService(ILendRecordRepository lendRecords)
+    {
+        _lendRecords = lendRecords;
+    }
+
+    public async Task<Result<LendRecord>> LendBookAsync(BookInstance bookInstance, Member member)
+    {
+        if (bookInstance.Status != BookInstanceStatus.OnShelf)
+            return Result<LendRecord>.Failure($"Book is not available for lending. Current status: {bookInstance.Status}.");
+
+        var code = await GenerateUniqueLendCodeAsync();
+        bookInstance.MarkAsLent();
+        var record = new LendRecord(bookInstance.Id, member.Id, code);
+        return Result<LendRecord>.Success(record);
+    }
+
+    public Task<Result> ReturnBookAsync(BookInstance bookInstance, LendRecord lendRecord)
+    {
+        if (bookInstance.Status != BookInstanceStatus.Lent)
+            return Task.FromResult(Result.Failure($"Book is not currently lent out. Current status: {bookInstance.Status}."));
+
+        bookInstance.ReturnToShelf();
+        lendRecord.MarkAsReturned();
+        return Task.FromResult(Result.Success());
+    }
+
+    private async Task<LendCode> GenerateUniqueLendCodeAsync()
+    {
+        const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
+        var random = Random.Shared;
+
+        while (true)
+        {
+            var code = new string(Enumerable.Range(0, 6).Select(_ => chars[random.Next(chars.Length)]).ToArray());
+            var lendCode = new LendCode(code);
+            if (!await _lendRecords.ExistsWithCodeAsync(lendCode))
+                return lendCode;
+        }
+    }
+}