diff --git a/backend/UniVerse.Domain/Entities/Achievement.cs b/backend/UniVerse.Domain/Entities/Achievement.cs
new file mode 100644
index 0000000..e17c88d
--- /dev/null
+++ b/backend/UniVerse.Domain/Entities/Achievement.cs
@@ -0,0 +1,16 @@
+namespace UniVerse.Domain.Entities;
+
+public class Achievement
+{
+    public int Id { get; set; }
+    public string Name { get; set; } = string.Empty;
+    public string? Description { get; set; }
+    public string? IconUrl { get; set; }
+    public int XpReward { get; set; }
+    public int CoinReward { get; set; }
+    public string? Condition { get; set; }
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+    // Navigation properties
+    public ICollection<UserAchievement> UserAchievements { get; set; } = new List<UserAchievement>();
+}
diff --git a/backend/UniVerse.Domain/Entities/CoinTransaction.cs b/backend/UniVerse.Domain/Entities/CoinTransaction.cs
new file mode 100644
index 0000000..457f2db
--- /dev/null
+++ b/backend/UniVerse.Domain/Entities/CoinTransaction.cs
@@ -0,0 +1,20 @@
+using UniVerse.Domain.Enums;
+
+namespace UniVerse.Domain.Entities;
+
+public class CoinTransaction
+{
+    public int Id { get; set; }
+    public int UserId { get; set; }
+    public int Amount { get; set; }
+    public CoinTransactionType Type { get; set; }
+    public int? ReviewId { get; set; }
+    public int? AchievementId { get; set; }
+    public string? Description { get; set; }
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+    // Navigation properties
+    public User User { get; set; } = null!;
+    public Review? Review { get; set; }
+    public Achievement? Achievement { get; set; }
+}
diff --git a/backend/UniVerse.Domain/Entities/Course.cs b/backend/UniVerse.Domain/Entities/Course.cs
new file mode 100644
index 0000000..3825db9
--- /dev/null
+++ b/backend/UniVerse.Domain/Entities/Course.cs
@@ -0,0 +1,16 @@
+namespace UniVerse.Domain.Entities;
+
+public class Course
+{
+    public int Id { get; set; }
+    public string Name { get; set; } = string.Empty;
+    public string? Description { get; set; }
+    public string? ExternalId { get; set; }
+    public bool IsSynced { get; set; }
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+    public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+
+    // Navigation properties
+    public ICollection<Lecture> Lectures { get; set; } = new List<Lecture>();
+    public ICollection<CourseTag> CourseTags { get; set; } = new List<CourseTag>();
+}
diff --git a/backend/UniVerse.Domain/Entities/CourseTag.cs b/backend/UniVerse.Domain/Entities/CourseTag.cs
new file mode 100644
index 0000000..4c15c51
--- /dev/null
+++ b/backend/UniVerse.Domain/Entities/CourseTag.cs
@@ -0,0 +1,12 @@
+namespace UniVerse.Domain.Entities;
+
+public class CourseTag
+{
+    public int Id { get; set; }
+    public int CourseId { get; set; }
+    public int TagId { get; set; }
+
+    // Navigation properties
+    public Course Course { get; set; } = null!;
+    public Tag Tag { get; set; } = null!;
+}
diff --git a/backend/UniVerse.Domain/Entities/Lecture.cs b/backend/UniVerse.Domain/Entities/Lecture.cs
new file mode 100644
index 0000000..37a732b
--- /dev/null
+++ b/backend/UniVerse.Domain/Entities/Lecture.cs
@@ -0,0 +1,29 @@
+using UniVerse.Domain.Enums;
+
+namespace UniVerse.Domain.Entities;
+
+public class Lecture
+{
+    public int Id { get; set; }
+    public int CourseId { get; set; }
+    public int? TeacherId { get; set; }
+    public int? LocationId { get; set; }
+    public string Title { get; set; } = string.Empty;
+    public string? Description { get; set; }
+    public LectureFormat Format { get; set; } = LectureFormat.Offline;
+    public DateTime StartsAt { get; set; }
+    public DateTime EndsAt { get; set; }
+    public bool IsOpen { get; set; } = true;
+    public int MaxEnrollments { get; set; }
+    public string? ExternalId { get; set; }
+    public string? OnlineUrl { get; set; }
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+    public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+
+    // Navigation properties
+    public Course Course { get; set; } = null!;
+    public User? Teacher { get; set; }
+    public Location? Location { get; set; }
+    public ICollection<LectureEnrollment> Enrollments { get; set; } = new List<LectureEnrollment>();
+    public ICollection<Review> Reviews { get; set; } = new List<Review>();
+}
diff --git a/backend/UniVerse.Domain/Entities/LectureEnrollment.cs b/backend/UniVerse.Domain/Entities/LectureEnrollment.cs
new file mode 100644
index 0000000..712f757
--- /dev/null
+++ b/backend/UniVerse.Domain/Entities/LectureEnrollment.cs
@@ -0,0 +1,14 @@
+namespace UniVerse.Domain.Entities;
+
+public class LectureEnrollment
+{
+    public int Id { get; set; }
+    public int LectureId { get; set; }
+    public int UserId { get; set; }
+    public bool Attended { get; set; }
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+    // Navigation properties
+    public Lecture Lecture { get; set; } = null!;
+    public User User { get; set; } = null!;
+}
diff --git a/backend/UniVerse.Domain/Entities/Location.cs b/backend/UniVerse.Domain/Entities/Location.cs
new file mode 100644
index 0000000..18c9339
--- /dev/null
+++ b/backend/UniVerse.Domain/Entities/Location.cs
@@ -0,0 +1,15 @@
+namespace UniVerse.Domain.Entities;
+
+public class Location
+{
+    public int Id { get; set; }
+    public string Name { get; set; } = string.Empty;
+    public string? Building { get; set; }
+    public string? Room { get; set; }
+    public string? Address { get; set; }
+    public string? ExternalId { get; set; }
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+    // Navigation properties
+    public ICollection<Lecture> Lectures { get; set; } = new List<Lecture>();
+}
diff --git a/backend/UniVerse.Domain/Entities/RefreshToken.cs b/backend/UniVerse.Domain/Entities/RefreshToken.cs
new file mode 100644
index 0000000..486ace4
--- /dev/null
+++ b/backend/UniVerse.Domain/Entities/RefreshToken.cs
@@ -0,0 +1,18 @@
+namespace UniVerse.Domain.Entities;
+
+public class RefreshToken
+{
+    public int Id { get; set; }
+    public int UserId { get; set; }
+    public string Token { get; set; } = string.Empty;
+    public DateTime ExpiresAt { get; set; }
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+    public DateTime? RevokedAt { get; set; }
+
+    public bool IsExpired => DateTime.UtcNow >= ExpiresAt;
+    public bool IsRevoked => RevokedAt != null;
+    public bool IsActive => !IsRevoked && !IsExpired;
+
+    // Navigation properties
+    public User User { get; set; } = null!;
+}
diff --git a/backend/UniVerse.Domain/Entities/Review.cs b/backend/UniVerse.Domain/Entities/Review.cs
new file mode 100644
index 0000000..9883dcd
--- /dev/null
+++ b/backend/UniVerse.Domain/Entities/Review.cs
@@ -0,0 +1,24 @@
+using UniVerse.Domain.Enums;
+
+namespace UniVerse.Domain.Entities;
+
+public class Review
+{
+    public int Id { get; set; }
+    public int LectureId { get; set; }
+    public int UserId { get; set; }
+    public ReviewRating Rating { get; set; }
+    public string? Text { get; set; }
+    public ReviewLlmStatus LlmStatus { get; set; } = ReviewLlmStatus.Pending;
+    public ReviewSentiment? Sentiment { get; set; }
+    public double? QualityScore { get; set; }
+    public bool? IsInformative { get; set; }
+    public string[]? LlmTags { get; set; }
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+    public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+
+    // Navigation properties
+    public Lecture Lecture { get; set; } = null!;
+    public User User { get; set; } = null!;
+    public ICollection<CoinTransaction> CoinTransactions { get; set; } = new List<CoinTransaction>();
+}
diff --git a/backend/UniVerse.Domain/Entities/StudentProfile.cs b/backend/UniVerse.Domain/Entities/StudentProfile.cs
new file mode 100644
index 0000000..1e47b76
--- /dev/null
+++ b/backend/UniVerse.Domain/Entities/StudentProfile.cs
@@ -0,0 +1,15 @@
+namespace UniVerse.Domain.Entities;
+
+public class StudentProfile
+{
+    public int Id { get; set; }
+    public int UserId { get; set; }
+    public string? StudentId { get; set; }
+    public string? GroupName { get; set; }
+    public int? EnrollmentYear { get; set; }
+    public string? Faculty { get; set; }
+    public string? Specialty { get; set; }
+
+    // Navigation properties
+    public User User { get; set; } = null!;
+}
diff --git a/backend/UniVerse.Domain/Entities/Tag.cs b/backend/UniVerse.Domain/Entities/Tag.cs
new file mode 100644
index 0000000..56b804a
--- /dev/null
+++ b/backend/UniVerse.Domain/Entities/Tag.cs
@@ -0,0 +1,17 @@
+using UniVerse.Domain.Enums;
+
+namespace UniVerse.Domain.Entities;
+
+public class Tag
+{
+    public int Id { get; set; }
+    public string Name { get; set; } = string.Empty;
+    public TagType Type { get; set; }
+    public int? ParentId { get; set; }
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+    // Navigation properties
+    public Tag? Parent { get; set; }
+    public ICollection<Tag> Children { get; set; } = new List<Tag>();
+    public ICollection<CourseTag> CourseTags { get; set; } = new List<CourseTag>();
+}
diff --git a/backend/UniVerse.Domain/Entities/TeacherProfile.cs b/backend/UniVerse.Domain/Entities/TeacherProfile.cs
new file mode 100644
index 0000000..8766ec0
--- /dev/null
+++ b/backend/UniVerse.Domain/Entities/TeacherProfile.cs
@@ -0,0 +1,14 @@
+namespace UniVerse.Domain.Entities;
+
+public class TeacherProfile
+{
+    public int Id { get; set; }
+    public int UserId { get; set; }
+    public string? Department { get; set; }
+    public string? Title { get; set; }
+    public string? Bio { get; set; }
+    public string? ModeusId { get; set; }
+
+    // Navigation properties
+    public User User { get; set; } = null!;
+}
diff --git a/backend/UniVerse.Domain/Entities/User.cs b/backend/UniVerse.Domain/Entities/User.cs
new file mode 100644
index 0000000..d7893de
--- /dev/null
+++ b/backend/UniVerse.Domain/Entities/User.cs
@@ -0,0 +1,27 @@
+using UniVerse.Domain.Enums;
+
+namespace UniVerse.Domain.Entities;
+
+public class User
+{
+    public int Id { get; set; }
+    public string Email { get; set; } = string.Empty;
+    public string? DisplayName { get; set; }
+    public string? AvatarUrl { get; set; }
+    public UserRole Role { get; set; } = UserRole.Student;
+    public bool IsActive { get; set; } = true;
+    public string? MicrosoftId { get; set; }
+    public int Xp { get; set; }
+    public int Coins { get; set; }
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+    public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+
+    // Navigation properties
+    public StudentProfile? StudentProfile { get; set; }
+    public TeacherProfile? TeacherProfile { get; set; }
+    public ICollection<LectureEnrollment> Enrollments { get; set; } = new List<LectureEnrollment>();
+    public ICollection<Review> Reviews { get; set; } = new List<Review>();
+    public ICollection<UserAchievement> UserAchievements { get; set; } = new List<UserAchievement>();
+    public ICollection<CoinTransaction> CoinTransactions { get; set; } = new List<CoinTransaction>();
+    public ICollection<RefreshToken> RefreshTokens { get; set; } = new List<RefreshToken>();
+}
diff --git a/backend/UniVerse.Domain/Entities/UserAchievement.cs b/backend/UniVerse.Domain/Entities/UserAchievement.cs
new file mode 100644
index 0000000..6f3fcd1
--- /dev/null
+++ b/backend/UniVerse.Domain/Entities/UserAchievement.cs
@@ -0,0 +1,13 @@
+namespace UniVerse.Domain.Entities;
+
+public class UserAchievement
+{
+    public int Id { get; set; }
+    public int UserId { get; set; }
+    public int AchievementId { get; set; }
+    public DateTime AwardedAt { get; set; } = DateTime.UtcNow;
+
+    // Navigation properties
+    public User User { get; set; } = null!;
+    public Achievement Achievement { get; set; } = null!;
+}
diff --git a/backend/UniVerse.Domain/Enums/CoinTransactionType.cs b/backend/UniVerse.Domain/Enums/CoinTransactionType.cs
new file mode 100644
index 0000000..0654fbe
--- /dev/null
+++ b/backend/UniVerse.Domain/Enums/CoinTransactionType.cs
@@ -0,0 +1,9 @@
+namespace UniVerse.Domain.Enums;
+
+public enum CoinTransactionType
+{
+    ReviewReward,
+    AchievementReward,
+    AttendanceReward,
+    AdminAdjustment
+}
diff --git a/backend/UniVerse.Domain/Enums/LectureFormat.cs b/backend/UniVerse.Domain/Enums/LectureFormat.cs
new file mode 100644
index 0000000..738d9d8
--- /dev/null
+++ b/backend/UniVerse.Domain/Enums/LectureFormat.cs
@@ -0,0 +1,7 @@
+namespace UniVerse.Domain.Enums;
+
+public enum LectureFormat
+{
+    Online,
+    Offline
+}
diff --git a/backend/UniVerse.Domain/Enums/ReviewLlmStatus.cs b/backend/UniVerse.Domain/Enums/ReviewLlmStatus.cs
new file mode 100644
index 0000000..937b0ef
--- /dev/null
+++ b/backend/UniVerse.Domain/Enums/ReviewLlmStatus.cs
@@ -0,0 +1,8 @@
+namespace UniVerse.Domain.Enums;
+
+public enum ReviewLlmStatus
+{
+    Pending,
+    Analyzed,
+    Rejected
+}
diff --git a/backend/UniVerse.Domain/Enums/ReviewRating.cs b/backend/UniVerse.Domain/Enums/ReviewRating.cs
new file mode 100644
index 0000000..e5d9f23
--- /dev/null
+++ b/backend/UniVerse.Domain/Enums/ReviewRating.cs
@@ -0,0 +1,8 @@
+namespace UniVerse.Domain.Enums;
+
+public enum ReviewRating
+{
+    Like,
+    Neutral,
+    Dislike
+}
diff --git a/backend/UniVerse.Domain/Enums/ReviewSentiment.cs b/backend/UniVerse.Domain/Enums/ReviewSentiment.cs
new file mode 100644
index 0000000..c3b6964
--- /dev/null
+++ b/backend/UniVerse.Domain/Enums/ReviewSentiment.cs
@@ -0,0 +1,8 @@
+namespace UniVerse.Domain.Enums;
+
+public enum ReviewSentiment
+{
+    Positive,
+    Neutral,
+    Negative
+}
diff --git a/backend/UniVerse.Domain/Enums/TagType.cs b/backend/UniVerse.Domain/Enums/TagType.cs
new file mode 100644
index 0000000..caef984
--- /dev/null
+++ b/backend/UniVerse.Domain/Enums/TagType.cs
@@ -0,0 +1,11 @@
+namespace UniVerse.Domain.Enums;
+
+public enum TagType
+{
+    Institute,
+    Faculty,
+    Subject,
+    Organization,
+    Topic,
+    Other
+}
diff --git a/backend/UniVerse.Domain/Enums/UserRole.cs b/backend/UniVerse.Domain/Enums/UserRole.cs
new file mode 100644
index 0000000..b664823
--- /dev/null
+++ b/backend/UniVerse.Domain/Enums/UserRole.cs
@@ -0,0 +1,8 @@
+namespace UniVerse.Domain.Enums;
+
+public enum UserRole
+{
+    Student,
+    Teacher,
+    Admin
+}
diff --git a/backend/UniVerse.Domain/Exceptions/ConflictException.cs b/backend/UniVerse.Domain/Exceptions/ConflictException.cs
new file mode 100644
index 0000000..500fe71
--- /dev/null
+++ b/backend/UniVerse.Domain/Exceptions/ConflictException.cs
@@ -0,0 +1,8 @@
+namespace UniVerse.Domain.Exceptions;
+
+public class ConflictException : Exception
+{
+    public ConflictException(string message) : base(message)
+    {
+    }
+}
diff --git a/backend/UniVerse.Domain/Exceptions/ForbiddenException.cs b/backend/UniVerse.Domain/Exceptions/ForbiddenException.cs
new file mode 100644
index 0000000..1188d6a
--- /dev/null
+++ b/backend/UniVerse.Domain/Exceptions/ForbiddenException.cs
@@ -0,0 +1,9 @@
+namespace UniVerse.Domain.Exceptions;
+
+public class ForbiddenException : Exception
+{
+    public ForbiddenException(string message = "You do not have permission to perform this action.")
+        : base(message)
+    {
+    }
+}
diff --git a/backend/UniVerse.Domain/Exceptions/NotFoundException.cs b/backend/UniVerse.Domain/Exceptions/NotFoundException.cs
new file mode 100644
index 0000000..9651bd1
--- /dev/null
+++ b/backend/UniVerse.Domain/Exceptions/NotFoundException.cs
@@ -0,0 +1,13 @@
+namespace UniVerse.Domain.Exceptions;
+
+public class NotFoundException : Exception
+{
+    public NotFoundException(string entityName, object key)
+        : base($"{entityName} with key '{key}' was not found.")
+    {
+    }
+
+    public NotFoundException(string message) : base(message)
+    {
+    }
+}
diff --git a/backend/UniVerse.Domain/UniVerse.Domain.csproj b/backend/UniVerse.Domain/UniVerse.Domain.csproj
new file mode 100644
index 0000000..e450d5e
--- /dev/null
+++ b/backend/UniVerse.Domain/UniVerse.Domain.csproj
@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0</TargetFramework>
+        <Nullable>enable</Nullable>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <RootNamespace>UniVerse.Domain</RootNamespace>
+    </PropertyGroup>
+
+</Project>