diff --git a/docs/IMPLEMENTATION_PLAN.md b/docs/IMPLEMENTATION_PLAN.md
index 3f84517..a02fb41 100644
--- a/docs/IMPLEMENTATION_PLAN.md
+++ b/docs/IMPLEMENTATION_PLAN.md
@@ -1699,7 +1699,7 @@ public enum CashBookEntryType { Income = 0, Expense = 1 }
 
 | ☐ | Phase | Bereich | Beschreibung | Dateien |
 |---|-------|---------|--------------|---------|
-| ☐ | K1 | Domain | Entities + Enums | 5 |
+| ✓ | K1 | Domain | Entities + Enums | 5 |
 | ☐ | K2 | Infrastructure | EF Configurations | 4 |
 | ☐ | K3 | Infrastructure | Migration | - |
 | ☐ | K4 | Security | Kassenwart Role + Policy | 4 |
diff --git a/src/Koogle.Domain/Entities/BookingCategory.cs b/src/Koogle.Domain/Entities/BookingCategory.cs
new file mode 100644
index 0000000..1971d59
--- /dev/null
+++ b/src/Koogle.Domain/Entities/BookingCategory.cs
@@ -0,0 +1,59 @@
+using Koogle.Domain.Enums;
+
+namespace Koogle.Domain.Entities;
+
+/// <summary>
+/// Represents a booking category for organizing cash book entries.
+/// </summary>
+public class BookingCategory : BaseEntity
+{
+    /// <summary>
+    /// Gets or sets the club identifier this category belongs to.
+    /// </summary>
+    public Guid ClubId { get; set; }
+
+    /// <summary>
+    /// Gets or sets the category name.
+    /// </summary>
+    public string Name { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the optional description.
+    /// </summary>
+    public string? Description { get; set; }
+
+    /// <summary>
+    /// Gets or sets the category type (Income or Expense).
+    /// </summary>
+    public BookingCategoryType CategoryType { get; set; }
+
+    /// <summary>
+    /// Gets or sets whether this is a system category (cannot be deleted).
+    /// </summary>
+    public bool IsSystemCategory { get; set; }
+
+    /// <summary>
+    /// Gets or sets the hex color code (#RRGGBB).
+    /// </summary>
+    public string? Color { get; set; }
+
+    /// <summary>
+    /// Gets or sets the MudBlazor icon name.
+    /// </summary>
+    public string? Icon { get; set; }
+
+    /// <summary>
+    /// Gets or sets whether this category is active (soft-deactivation).
+    /// </summary>
+    public bool IsActive { get; set; } = true;
+
+    /// <summary>
+    /// Gets or sets the club navigation property.
+    /// </summary>
+    public Club Club { get; set; } = null!;
+
+    /// <summary>
+    /// Gets or sets the cash book entries in this category.
+    /// </summary>
+    public ICollection<CashBookEntry> CashBookEntries { get; set; } = [];
+}
diff --git a/src/Koogle.Domain/Entities/CashBookEntry.cs b/src/Koogle.Domain/Entities/CashBookEntry.cs
new file mode 100644
index 0000000..bc7b183
--- /dev/null
+++ b/src/Koogle.Domain/Entities/CashBookEntry.cs
@@ -0,0 +1,74 @@
+using Koogle.Domain.Enums;
+
+namespace Koogle.Domain.Entities;
+
+/// <summary>
+/// Represents a cash book entry (income or expense transaction).
+/// </summary>
+public class CashBookEntry : BaseEntity
+{
+    /// <summary>
+    /// Gets or sets the club identifier this entry belongs to.
+    /// </summary>
+    public Guid ClubId { get; set; }
+
+    /// <summary>
+    /// Gets or sets the booking category identifier.
+    /// </summary>
+    public Guid CategoryId { get; set; }
+
+    /// <summary>
+    /// Gets or sets the entry type (Income or Expense).
+    /// </summary>
+    public CashBookEntryType EntryType { get; set; }
+
+    /// <summary>
+    /// Gets or sets the amount (always positive).
+    /// </summary>
+    public decimal Amount { get; set; }
+
+    /// <summary>
+    /// Gets or sets the booking date (freely selectable).
+    /// </summary>
+    public DateTime BookingDate { get; set; }
+
+    /// <summary>
+    /// Gets or sets the optional comment.
+    /// </summary>
+    public string? Comment { get; set; }
+
+    /// <summary>
+    /// Gets or sets the receipt reference.
+    /// </summary>
+    public string? ReceiptReference { get; set; }
+
+    /// <summary>
+    /// Gets or sets the optional day identifier (for penalty bookings).
+    /// </summary>
+    public Guid? DayId { get; set; }
+
+    /// <summary>
+    /// Gets or sets the optional person identifier.
+    /// </summary>
+    public Guid? PersonId { get; set; }
+
+    /// <summary>
+    /// Gets or sets the club navigation property.
+    /// </summary>
+    public Club Club { get; set; } = null!;
+
+    /// <summary>
+    /// Gets or sets the category navigation property.
+    /// </summary>
+    public BookingCategory Category { get; set; } = null!;
+
+    /// <summary>
+    /// Gets or sets the optional day navigation property.
+    /// </summary>
+    public Day? Day { get; set; }
+
+    /// <summary>
+    /// Gets or sets the optional person navigation property.
+    /// </summary>
+    public Person? Person { get; set; }
+}
diff --git a/src/Koogle.Domain/Entities/Club.cs b/src/Koogle.Domain/Entities/Club.cs
index a8a86af..952005b 100644
--- a/src/Koogle.Domain/Entities/Club.cs
+++ b/src/Koogle.Domain/Entities/Club.cs
@@ -33,6 +33,16 @@ namespace Koogle.Domain.Entities
         /// </summary>
         public string? SenderEmail { get; set; }
 
+        /// <summary>
+        /// Initial cash book balance (opening balance).
+        /// </summary>
+        public decimal InitialBalance { get; set; }
+
+        /// <summary>
+        /// Monthly membership fee amount.
+        /// </summary>
+        public decimal MonthlyMembershipFee { get; set; }
+
         /// <summary>
         /// Persons in this club.
         /// </summary>
@@ -62,5 +72,15 @@ namespace Koogle.Domain.Entities
         /// Tokens for anonymous GIF submissions.
         /// </summary>
         public ICollection<GifSubmissionToken> GifSubmissionTokens { get; set; } = new List<GifSubmissionToken>();
+
+        /// <summary>
+        /// Booking categories for cash book organization.
+        /// </summary>
+        public ICollection<BookingCategory> BookingCategories { get; set; } = new List<BookingCategory>();
+
+        /// <summary>
+        /// Cash book entries for this club.
+        /// </summary>
+        public ICollection<CashBookEntry> CashBookEntries { get; set; } = new List<CashBookEntry>();
     }
 }
diff --git a/src/Koogle.Domain/Enums/BookingCategoryType.cs b/src/Koogle.Domain/Enums/BookingCategoryType.cs
new file mode 100644
index 0000000..fe2370a
--- /dev/null
+++ b/src/Koogle.Domain/Enums/BookingCategoryType.cs
@@ -0,0 +1,17 @@
+namespace Koogle.Domain.Enums;
+
+/// <summary>
+/// Defines the type of booking category for cash book entries.
+/// </summary>
+public enum BookingCategoryType
+{
+    /// <summary>
+    /// Income category (e.g., game penalties, membership fees).
+    /// </summary>
+    Income = 0,
+
+    /// <summary>
+    /// Expense category (e.g., lane rental, materials).
+    /// </summary>
+    Expense = 1
+}
diff --git a/src/Koogle.Domain/Enums/CashBookEntryType.cs b/src/Koogle.Domain/Enums/CashBookEntryType.cs
new file mode 100644
index 0000000..87e5a86
--- /dev/null
+++ b/src/Koogle.Domain/Enums/CashBookEntryType.cs
@@ -0,0 +1,17 @@
+namespace Koogle.Domain.Enums;
+
+/// <summary>
+/// Defines the type of cash book entry (income or expense).
+/// </summary>
+public enum CashBookEntryType
+{
+    /// <summary>
+    /// Income entry (money coming in).
+    /// </summary>
+    Income = 0,
+
+    /// <summary>
+    /// Expense entry (money going out).
+    /// </summary>
+    Expense = 1
+}