diff --git a/docs/IMPLEMENTATION_PLAN.md b/docs/IMPLEMENTATION_PLAN.md
index 63acc71..fdce0cf 100644
--- a/docs/IMPLEMENTATION_PLAN.md
+++ b/docs/IMPLEMENTATION_PLAN.md
@@ -1704,7 +1704,7 @@ public enum CashBookEntryType { Income = 0, Expense = 1 }
 | ✓ | K3 | Infrastructure | Migration | - |
 | ✓ | K4 | Security | Kassenwart Role + Policy | 4 |
 | ✓ | K5 | Infrastructure | Repositories | 5 |
-| ☐ | K6 | Application | DTOs | 3 |
+| ✓ | K6 | Application | DTOs | 3 |
 | ☐ | K7 | Application | Service Interfaces | 2 |
 | ☐ | K8 | Application | Service Implementations | 4 |
 | ☐ | K9 | Application | Category Seeder | 1 |
diff --git a/src/Koogle.Application/DTOs/BookingCategoryDto.cs b/src/Koogle.Application/DTOs/BookingCategoryDto.cs
new file mode 100644
index 0000000..75e19f7
--- /dev/null
+++ b/src/Koogle.Application/DTOs/BookingCategoryDto.cs
@@ -0,0 +1,43 @@
+using Koogle.Domain.Enums;
+
+namespace Koogle.Application.DTOs;
+
+/// <summary>
+/// Data transfer object representing a booking category.
+/// </summary>
+public record BookingCategoryDto
+{
+    public Guid Id { get; init; }
+    public string Name { get; init; } = string.Empty;
+    public string? Description { get; init; }
+    public BookingCategoryType CategoryType { get; init; }
+    public bool IsSystemCategory { get; init; }
+    public string? Color { get; init; }
+    public string? Icon { get; init; }
+    public bool IsActive { get; init; }
+}
+
+/// <summary>
+/// Data transfer object for creating a new booking category.
+/// </summary>
+public record CreateBookingCategoryDto
+{
+    public string Name { get; init; } = string.Empty;
+    public string? Description { get; init; }
+    public BookingCategoryType CategoryType { get; init; }
+    public string? Color { get; init; }
+    public string? Icon { get; init; }
+}
+
+/// <summary>
+/// Data transfer object for updating an existing booking category.
+/// </summary>
+public record UpdateBookingCategoryDto
+{
+    public Guid Id { get; init; }
+    public string Name { get; init; } = string.Empty;
+    public string? Description { get; init; }
+    public string? Color { get; init; }
+    public string? Icon { get; init; }
+    public bool IsActive { get; init; }
+}
diff --git a/src/Koogle.Application/DTOs/CashBookEntryDto.cs b/src/Koogle.Application/DTOs/CashBookEntryDto.cs
new file mode 100644
index 0000000..5e92176
--- /dev/null
+++ b/src/Koogle.Application/DTOs/CashBookEntryDto.cs
@@ -0,0 +1,49 @@
+using Koogle.Domain.Enums;
+
+namespace Koogle.Application.DTOs;
+
+/// <summary>
+/// Data transfer object representing a cash book entry.
+/// </summary>
+public record CashBookEntryDto
+{
+    public Guid Id { get; init; }
+    public Guid CategoryId { get; init; }
+    public string CategoryName { get; init; } = string.Empty;
+    public CashBookEntryType EntryType { get; init; }
+    public decimal Amount { get; init; }
+    public DateTime BookingDate { get; init; }
+    public string? Comment { get; init; }
+    public string? ReceiptReference { get; init; }
+    public Guid? DayId { get; init; }
+    public Guid? PersonId { get; init; }
+    public string? PersonName { get; init; }
+    public DateTime CreatedAt { get; init; }
+}
+
+/// <summary>
+/// Data transfer object for creating a new cash book entry.
+/// </summary>
+public record CreateCashBookEntryDto
+{
+    public Guid CategoryId { get; init; }
+    public CashBookEntryType EntryType { get; init; }
+    public decimal Amount { get; init; }
+    public DateTime BookingDate { get; init; }
+    public string? Comment { get; init; }
+    public string? ReceiptReference { get; init; }
+    public Guid? PersonId { get; init; }
+}
+
+/// <summary>
+/// Data transfer object for updating an existing cash book entry.
+/// </summary>
+public record UpdateCashBookEntryDto
+{
+    public Guid Id { get; init; }
+    public Guid CategoryId { get; init; }
+    public decimal Amount { get; init; }
+    public DateTime BookingDate { get; init; }
+    public string? Comment { get; init; }
+    public string? ReceiptReference { get; init; }
+}
diff --git a/src/Koogle.Application/DTOs/CashBookReportDto.cs b/src/Koogle.Application/DTOs/CashBookReportDto.cs
new file mode 100644
index 0000000..157c639
--- /dev/null
+++ b/src/Koogle.Application/DTOs/CashBookReportDto.cs
@@ -0,0 +1,38 @@
+namespace Koogle.Application.DTOs;
+
+/// <summary>
+/// Data transfer object for a cash book report.
+/// </summary>
+public record CashBookReportDto
+{
+    public DateTime ReportStart { get; init; }
+    public DateTime ReportEnd { get; init; }
+    public decimal OpeningBalance { get; init; }
+    public decimal ClosingBalance { get; init; }
+    public decimal TotalIncome { get; init; }
+    public decimal TotalExpense { get; init; }
+    public List<CategorySummaryDto> IncomeByCategory { get; init; } = [];
+    public List<CategorySummaryDto> ExpenseByCategory { get; init; } = [];
+    public List<CashBookEntryDto> Entries { get; init; } = [];
+}
+
+/// <summary>
+/// Summary of entries for a specific category.
+/// </summary>
+public record CategorySummaryDto
+{
+    public Guid CategoryId { get; init; }
+    public string CategoryName { get; init; } = string.Empty;
+    public decimal Total { get; init; }
+    public int Count { get; init; }
+}
+
+/// <summary>
+/// Data transfer object for creating membership fee entries.
+/// </summary>
+public record CreateMembershipFeesDto
+{
+    public int Year { get; init; }
+    public int Month { get; init; }
+    public decimal? OverrideAmount { get; init; }
+}