diff --git a/backend/src/main/resources/db/migration/V12__add_document_comments.sql b/backend/src/main/resources/db/migration/V12__add_document_comments.sql
new file mode 100644
index 00000000..12e3b843
--- /dev/null
+++ b/backend/src/main/resources/db/migration/V12__add_document_comments.sql
@@ -0,0 +1,15 @@
+CREATE TABLE document_comments (
+    id              UUID         PRIMARY KEY DEFAULT gen_random_uuid(),
+    document_id     UUID         NOT NULL REFERENCES documents(id) ON DELETE CASCADE,
+    annotation_id   UUID         REFERENCES document_annotations(id) ON DELETE CASCADE,
+    parent_id       UUID         REFERENCES document_comments(id) ON DELETE CASCADE,
+    author_id       UUID         REFERENCES users(id) ON DELETE SET NULL,
+    author_name     VARCHAR(200) NOT NULL,
+    content         TEXT         NOT NULL,
+    created_at      TIMESTAMP    NOT NULL DEFAULT now(),
+    updated_at      TIMESTAMP    NOT NULL DEFAULT now()
+);
+
+CREATE INDEX idx_dc_document   ON document_comments(document_id);
+CREATE INDEX idx_dc_annotation ON document_comments(annotation_id);
+CREATE INDEX idx_dc_parent     ON document_comments(parent_id);
diff --git a/backend/src/test/java/org/raddatz/familienarchiv/service/CommentServiceTest.java b/backend/src/test/java/org/raddatz/familienarchiv/service/CommentServiceTest.java
new file mode 100644
index 00000000..13d1906c
--- /dev/null
+++ b/backend/src/test/java/org/raddatz/familienarchiv/service/CommentServiceTest.java
@@ -0,0 +1,249 @@
+package org.raddatz.familienarchiv.service;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.raddatz.familienarchiv.exception.DomainException;
+import org.raddatz.familienarchiv.model.AppUser;
+import org.raddatz.familienarchiv.model.DocumentComment;
+import org.raddatz.familienarchiv.model.UserGroup;
+import org.raddatz.familienarchiv.repository.CommentRepository;
+
+import java.time.LocalDateTime;
+import java.util.List;
+import java.util.Optional;
+import java.util.Set;
+import java.util.UUID;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+import static org.springframework.http.HttpStatus.FORBIDDEN;
+import static org.springframework.http.HttpStatus.NOT_FOUND;
+
+@ExtendWith(MockitoExtension.class)
+class CommentServiceTest {
+
+    @Mock CommentRepository commentRepository;
+    @InjectMocks CommentService commentService;
+
+    // ─── postComment ──────────────────────────────────────────────────────────
+
+    @Test
+    void postComment_capturesAuthorNameAtWriteTime() {
+        UUID docId = UUID.randomUUID();
+        AppUser author = AppUser.builder()
+                .id(UUID.randomUUID()).username("hans").firstName("Hans").lastName("Müller").build();
+        DocumentComment saved = DocumentComment.builder()
+                .id(UUID.randomUUID()).documentId(docId).authorName("Hans Müller").content("Test").build();
+        when(commentRepository.save(any())).thenReturn(saved);
+
+        DocumentComment result = commentService.postComment(docId, null, "Test", author);
+
+        assertThat(result.getAuthorName()).isEqualTo("Hans Müller");
+    }
+
+    @Test
+    void postComment_fallsBackToUsername_whenNamesAreBlank() {
+        UUID docId = UUID.randomUUID();
+        AppUser author = AppUser.builder().id(UUID.randomUUID()).username("hans42").build();
+        DocumentComment saved = DocumentComment.builder()
+                .id(UUID.randomUUID()).documentId(docId).authorName("hans42").content("Test").build();
+        when(commentRepository.save(any())).thenReturn(saved);
+
+        DocumentComment result = commentService.postComment(docId, null, "Test", author);
+
+        assertThat(result.getAuthorName()).isEqualTo("hans42");
+    }
+
+    // ─── replyToComment ───────────────────────────────────────────────────────
+
+    @Test
+    void replyToComment_throwsNotFound_whenTargetCommentMissing() {
+        UUID docId = UUID.randomUUID();
+        UUID commentId = UUID.randomUUID();
+        AppUser author = AppUser.builder().id(UUID.randomUUID()).username("anna").build();
+        when(commentRepository.findById(commentId)).thenReturn(Optional.empty());
+
+        assertThatThrownBy(() -> commentService.replyToComment(docId, commentId, "Reply", author))
+                .isInstanceOf(DomainException.class)
+                .satisfies(e -> assertThat(((DomainException) e).getStatus()).isEqualTo(NOT_FOUND));
+
+        verify(commentRepository, never()).save(any());
+    }
+
+    @Test
+    void replyToComment_resolvesToRootParent_whenReplyingToAReply() {
+        UUID docId = UUID.randomUUID();
+        UUID rootId = UUID.randomUUID();
+        UUID replyId = UUID.randomUUID();
+        AppUser author = AppUser.builder().id(UUID.randomUUID()).username("anna").build();
+
+        DocumentComment root = DocumentComment.builder()
+                .id(rootId).documentId(docId).parentId(null).content("Root").authorName("Hans").build();
+        DocumentComment existingReply = DocumentComment.builder()
+                .id(replyId).documentId(docId).parentId(rootId).content("Reply1").authorName("Anna").build();
+
+        when(commentRepository.findById(replyId)).thenReturn(Optional.of(existingReply));
+        when(commentRepository.findById(rootId)).thenReturn(Optional.of(root));
+        DocumentComment saved = DocumentComment.builder()
+                .id(UUID.randomUUID()).documentId(docId).parentId(rootId).content("Reply2").authorName("anna").build();
+        when(commentRepository.save(any())).thenReturn(saved);
+
+        DocumentComment result = commentService.replyToComment(docId, replyId, "Reply2", author);
+
+        assertThat(result.getParentId()).isEqualTo(rootId);
+    }
+
+    @Test
+    void replyToComment_usesDirectComment_whenReplyingToTopLevel() {
+        UUID docId = UUID.randomUUID();
+        UUID rootId = UUID.randomUUID();
+        AppUser author = AppUser.builder().id(UUID.randomUUID()).username("anna").build();
+
+        DocumentComment root = DocumentComment.builder()
+                .id(rootId).documentId(docId).parentId(null).content("Root").authorName("Hans").build();
+
+        when(commentRepository.findById(rootId)).thenReturn(Optional.of(root));
+        DocumentComment saved = DocumentComment.builder()
+                .id(UUID.randomUUID()).documentId(docId).parentId(rootId).content("Reply").authorName("anna").build();
+        when(commentRepository.save(any())).thenReturn(saved);
+
+        DocumentComment result = commentService.replyToComment(docId, rootId, "Reply", author);
+
+        assertThat(result.getParentId()).isEqualTo(rootId);
+    }
+
+    // ─── editComment ──────────────────────────────────────────────────────────
+
+    @Test
+    void editComment_throwsForbidden_whenNotAuthor() {
+        UUID docId = UUID.randomUUID();
+        UUID commentId = UUID.randomUUID();
+        UUID ownerId = UUID.randomUUID();
+        AppUser other = AppUser.builder().id(UUID.randomUUID()).username("other").build();
+
+        DocumentComment comment = DocumentComment.builder()
+                .id(commentId).documentId(docId).authorId(ownerId).content("Original").authorName("Hans").build();
+        when(commentRepository.findById(commentId)).thenReturn(Optional.of(comment));
+
+        assertThatThrownBy(() -> commentService.editComment(docId, commentId, "Changed", other))
+                .isInstanceOf(DomainException.class)
+                .satisfies(e -> assertThat(((DomainException) e).getStatus()).isEqualTo(FORBIDDEN));
+
+        verify(commentRepository, never()).save(any());
+    }
+
+    @Test
+    void editComment_updatesContent_whenAuthor() {
+        UUID docId = UUID.randomUUID();
+        UUID commentId = UUID.randomUUID();
+        UUID authorId = UUID.randomUUID();
+        AppUser author = AppUser.builder().id(authorId).username("hans").build();
+        LocalDateTime created = LocalDateTime.now().minusMinutes(5);
+
+        DocumentComment comment = DocumentComment.builder()
+                .id(commentId).documentId(docId).authorId(authorId)
+                .content("Original").authorName("Hans").createdAt(created).build();
+        when(commentRepository.findById(commentId)).thenReturn(Optional.of(comment));
+        when(commentRepository.save(any())).thenAnswer(inv -> inv.getArgument(0));
+
+        DocumentComment result = commentService.editComment(docId, commentId, "Updated", author);
+
+        assertThat(result.getContent()).isEqualTo("Updated");
+        assertThat(result.getCreatedAt()).isEqualTo(created);
+    }
+
+    // ─── deleteComment ────────────────────────────────────────────────────────
+
+    @Test
+    void deleteComment_throwsForbidden_whenNotAuthorAndNotAdmin() {
+        UUID docId = UUID.randomUUID();
+        UUID commentId = UUID.randomUUID();
+        UUID ownerId = UUID.randomUUID();
+        AppUser other = AppUser.builder().id(UUID.randomUUID()).username("other").build();
+
+        DocumentComment comment = DocumentComment.builder()
+                .id(commentId).documentId(docId).authorId(ownerId).authorName("Hans").content("X").build();
+        when(commentRepository.findById(commentId)).thenReturn(Optional.of(comment));
+
+        assertThatThrownBy(() -> commentService.deleteComment(docId, commentId, other))
+                .isInstanceOf(DomainException.class)
+                .satisfies(e -> assertThat(((DomainException) e).getStatus()).isEqualTo(FORBIDDEN));
+
+        verify(commentRepository, never()).delete(any());
+    }
+
+    @Test
+    void deleteComment_succeeds_whenAuthor() {
+        UUID docId = UUID.randomUUID();
+        UUID commentId = UUID.randomUUID();
+        UUID authorId = UUID.randomUUID();
+        AppUser author = AppUser.builder().id(authorId).username("hans").build();
+
+        DocumentComment comment = DocumentComment.builder()
+                .id(commentId).documentId(docId).authorId(authorId).authorName("Hans").content("X").build();
+        when(commentRepository.findById(commentId)).thenReturn(Optional.of(comment));
+
+        commentService.deleteComment(docId, commentId, author);
+
+        verify(commentRepository).delete(comment);
+    }
+
+    @Test
+    void deleteComment_succeeds_whenAdmin() {
+        UUID docId = UUID.randomUUID();
+        UUID commentId = UUID.randomUUID();
+        UUID ownerId = UUID.randomUUID();
+        AppUser admin = buildAdmin();
+
+        DocumentComment comment = DocumentComment.builder()
+                .id(commentId).documentId(docId).authorId(ownerId).authorName("Hans").content("X").build();
+        when(commentRepository.findById(commentId)).thenReturn(Optional.of(comment));
+
+        commentService.deleteComment(docId, commentId, admin);
+
+        verify(commentRepository).delete(comment);
+    }
+
+    // ─── getCommentsForDocument ───────────────────────────────────────────────
+
+    @Test
+    void getCommentsForDocument_returnsRootsWithRepliesAttached() {
+        UUID docId = UUID.randomUUID();
+        UUID rootId = UUID.randomUUID();
+
+        DocumentComment root = DocumentComment.builder()
+                .id(rootId).documentId(docId).authorName("Hans").content("Root").build();
+        DocumentComment reply = DocumentComment.builder()
+                .id(UUID.randomUUID()).documentId(docId).parentId(rootId).authorName("Anna").content("Reply").build();
+
+        when(commentRepository.findByDocumentIdAndAnnotationIdIsNullAndParentIdIsNull(docId))
+                .thenReturn(List.of(root));
+        when(commentRepository.findByParentId(rootId)).thenReturn(List.of(reply));
+
+        List<DocumentComment> result = commentService.getCommentsForDocument(docId);
+
+        assertThat(result).hasSize(1);
+        assertThat(result.get(0).getReplies()).containsExactly(reply);
+    }
+
+    // ─── helpers ──────────────────────────────────────────────────────────────
+
+    private AppUser buildAdmin() {
+        return AppUser.builder()
+                .id(UUID.randomUUID())
+                .username("admin")
+                .groups(Set.of(UserGroup.builder()
+                        .id(UUID.randomUUID())
+                        .name("admins")
+                        .permissions(Set.of("ADMIN"))
+                        .build()))
+                .build();
+    }
+}