diff --git a/src/lib/components/CommentNode.svelte b/src/lib/components/CommentNode.svelte
new file mode 100644
index 0000000..92bda2a
--- /dev/null
+++ b/src/lib/components/CommentNode.svelte
@@ -0,0 +1,262 @@
+<script lang="ts">
+	import { mutation } from "$lib/api";
+	import { getToken, currentUser, isAuthenticated } from "$lib/stores/auth";
+	import { get } from "svelte/store";
+	import { timeAgo } from "$lib/time";
+	import CommentNode from "./CommentNode.svelte";
+	import type { CommentDoc } from "$lib/types";
+
+	interface CommentTreeNode {
+		comment: CommentDoc;
+		children: CommentTreeNode[];
+	}
+
+	let {
+		comment,
+		children = [],
+		targetType,
+		targetId,
+		reload,
+	}: {
+		comment: CommentDoc;
+		children?: CommentTreeNode[];
+		targetType: "note" | "question";
+		targetId: string;
+		reload: () => Promise<void>;
+	} = $props();
+
+	let collapsed = $state(false);
+	let replying = $state(false);
+	let replyText = $state("");
+	let replyLoading = $state(false);
+	let replyError = $state("");
+
+	let editing = $state(false);
+	let editText = $state("");
+	let editLoading = $state(false);
+	let editError = $state("");
+
+	let deleteLoading = $state(false);
+
+	const isMine = $derived(get(currentUser)?._id === comment.authorId);
+	const edited = $derived(comment.updatedAt != null && comment.updatedAt > comment.createdAt);
+
+	async function submitReply() {
+		if (!replyText.trim()) return;
+		const token = getToken();
+		if (!token) {
+			replyError = "Please sign in to reply";
+			return;
+		}
+
+		replyLoading = true;
+		replyError = "";
+		try {
+			await mutation(
+				targetType === "note" ? "comments:createOnNote" : "comments:createOnQuestion",
+				targetType === "note"
+					? {
+							token,
+							content: replyText.trim(),
+							parentId: targetId,
+							parentCommentId: comment._id,
+						}
+					: {
+							token,
+							content: replyText.trim(),
+							questionId: targetId,
+							parentCommentId: comment._id,
+						},
+			);
+			replyText = "";
+			replying = false;
+			collapsed = false;
+			await reload();
+		} catch (err: any) {
+			replyError = err.message ?? "Failed to post reply";
+		} finally {
+			replyLoading = false;
+		}
+	}
+
+	function startEdit() {
+		editText = comment.content;
+		editError = "";
+		editing = true;
+	}
+
+	function cancelEdit() {
+		editing = false;
+		editError = "";
+	}
+
+	async function saveEdit() {
+		if (!editText.trim()) {
+			editError = "Comment cannot be empty";
+			return;
+		}
+		const token = getToken();
+		if (!token) {
+			editError = "You must be signed in";
+			return;
+		}
+
+		editLoading = true;
+		editError = "";
+		try {
+			await mutation("comments:update", { token, id: comment._id, content: editText.trim() });
+			editing = false;
+			await reload();
+		} catch (err: any) {
+			editError = err.message ?? "Failed to save";
+		} finally {
+			editLoading = false;
+		}
+	}
+
+	async function remove() {
+		const token = getToken();
+		if (!token) return;
+		if (!confirm("Delete this comment and its replies?")) return;
+
+		deleteLoading = true;
+		try {
+			await mutation("comments:remove", { token, id: comment._id });
+			await reload();
+		} catch (err: any) {
+			alert(err.message ?? "Failed to delete");
+		} finally {
+			deleteLoading = false;
+		}
+	}
+</script>
+
+<div class="py-3">
+	<div class="flex gap-3">
+		<div class="min-w-0 flex-1">
+			<p class="kicker">
+				{comment.authorName} · <span class="text-faint">{timeAgo(comment.createdAt)}</span>
+				{#if edited}
+					· Edited
+				{/if}
+			</p>
+
+			{#if editing}
+				<textarea bind:value={editText} rows={3} class="field mt-2 resize-y"></textarea>
+				<div class="mt-2 flex items-center gap-3">
+					<button
+						type="button"
+						onclick={saveEdit}
+						disabled={editLoading}
+						class="btn-primary"
+					>
+						{editLoading ? "Saving..." : "Save"}
+					</button>
+					<button type="button" onclick={cancelEdit} class="kicker hover:text-ink">
+						Cancel
+					</button>
+					<span class="text-primary text-xs">{editError}</span>
+				</div>
+			{:else}
+				<p class="text-ink mt-1 text-sm leading-relaxed whitespace-pre-wrap">
+					{comment.content}
+				</p>
+			{/if}
+
+			<div class="mt-2 flex items-center gap-3">
+				{#if get(isAuthenticated)}
+					<button
+						type="button"
+						onclick={() => {
+							replying = !replying;
+							replyError = "";
+						}}
+						class="text-secondary hover:text-secondary-dark text-xs"
+					>
+						Reply
+					</button>
+				{/if}
+				{#if isMine}
+					{#if !editing}
+						<button
+							type="button"
+							onclick={startEdit}
+							class="text-secondary hover:text-secondary-dark text-xs"
+						>
+							Edit
+						</button>
+					{/if}
+					<button
+						type="button"
+						onclick={remove}
+						disabled={deleteLoading}
+						class="text-primary hover:text-primary-dark text-xs"
+					>
+						{deleteLoading ? "Deleting..." : "Delete"}
+					</button>
+				{/if}
+			</div>
+
+			{#if replying}
+				<div class="mt-3">
+					<textarea
+						bind:value={replyText}
+						rows={2}
+						placeholder="Reply..."
+						class="field resize-y"></textarea>
+					<div class="mt-2 flex items-center gap-3">
+						<button
+							type="button"
+							onclick={submitReply}
+							disabled={replyLoading || !replyText.trim()}
+							class="btn-primary"
+						>
+							{replyLoading ? "Posting..." : "Post reply"}
+						</button>
+						<button
+							type="button"
+							onclick={() => (replying = false)}
+							class="kicker hover:text-ink"
+						>
+							Cancel
+						</button>
+						<span class="text-primary text-xs">{replyError}</span>
+					</div>
+				</div>
+			{/if}
+		</div>
+	</div>
+
+	{#if children.length > 0}
+		<div class="mt-1 flex">
+			<button
+				type="button"
+				class="thread-branch"
+				onclick={() => (collapsed = !collapsed)}
+				title={collapsed ? "Show replies" : "Hide replies"}
+				aria-label={collapsed ? "Show replies" : "Hide replies"}
+			></button>
+			<div class="min-w-0 flex-1 pl-3">
+				{#if collapsed}
+					<button
+						type="button"
+						onclick={() => (collapsed = false)}
+						class="text-muted hover:text-ink py-1 text-xs"
+					>
+						Show {children.length} repl{children.length === 1 ? "y" : "ies"}
+					</button>
+				{:else}
+					{#each children as child}
+						<CommentNode
+							comment={child.comment}
+							children={child.children}
+							{targetType}
+							{targetId}
+							{reload}
+						/>
+					{/each}
+				{/if}
+			</div>
+		</div>
+	{/if}
+</div>
diff --git a/src/lib/components/CommentThread.svelte b/src/lib/components/CommentThread.svelte
new file mode 100644
index 0000000..a931089
--- /dev/null
+++ b/src/lib/components/CommentThread.svelte
@@ -0,0 +1,54 @@
+<script lang="ts">
+	import CommentNode from "./CommentNode.svelte";
+	import type { CommentDoc } from "$lib/types";
+
+	interface CommentTreeNode {
+		comment: CommentDoc;
+		children: CommentTreeNode[];
+	}
+
+	let {
+		comments = [],
+		targetType,
+		targetId,
+		reload,
+	}: {
+		comments?: CommentDoc[];
+		targetType: "note" | "question";
+		targetId: string;
+		reload: () => Promise<void>;
+	} = $props();
+
+	const roots = $derived.by(() => {
+		const map = new Map<string, CommentTreeNode>();
+		for (const comment of comments) {
+			map.set(comment._id, { comment, children: [] });
+		}
+		const result: CommentTreeNode[] = [];
+		for (const comment of comments) {
+			const node = map.get(comment._id)!;
+			if (comment.parentCommentId && map.has(comment.parentCommentId)) {
+				map.get(comment.parentCommentId)!.children.push(node);
+			} else {
+				result.push(node);
+			}
+		}
+		return result;
+	});
+</script>
+
+<div>
+	{#each roots as root}
+		<div class="border-rule border-t">
+			<CommentNode
+				comment={root.comment}
+				children={root.children}
+				{targetType}
+				{targetId}
+				{reload}
+			/>
+		</div>
+	{:else}
+		<p class="text-muted text-sm">No {targetType === "note" ? "comments" : "answers"} yet.</p>
+	{/each}
+</div>
diff --git a/src/lib/server/api.ts b/src/lib/server/api.ts
index e50aab1..8485bb6 100644
--- a/src/lib/server/api.ts
+++ b/src/lib/server/api.ts
@@ -529,7 +529,7 @@ function questionsUpdate(
 // ---- comments ----
 
 const COMMENT_COLUMNS =
-	"id AS _id, content, authorId, authorName, parentId, questionId, parentCommentId, createdAt";
+	"id AS _id, content, authorId, authorName, parentId, questionId, parentCommentId, createdAt, updatedAt";
 
 function commentsCreateOnNote(
 	db: Db,
@@ -537,9 +537,10 @@ function commentsCreateOnNote(
 ) {
 	const user = requireAuth(db, args.token);
 	const id = newId();
+	const now = Date.now();
 	db.prepare(
-		`INSERT INTO comments (id, content, authorId, authorName, parentId, parentCommentId, createdAt)
-     VALUES (?, ?, ?, ?, ?, ?, ?)`,
+		`INSERT INTO comments (id, content, authorId, authorName, parentId, parentCommentId, createdAt, updatedAt)
+     VALUES (?, ?, ?, ?, ?, ?, ?, ?)`,
 	).run(
 		id,
 		args.content,
@@ -547,7 +548,8 @@ function commentsCreateOnNote(
 		user.name,
 		args.parentId,
 		args.parentCommentId ?? null,
-		Date.now(),
+		now,
+		now,
 	);
 	db.prepare("UPDATE notes SET commentCount = commentCount + 1 WHERE id = ?").run(args.parentId);
 	return id;
@@ -559,9 +561,10 @@ function commentsCreateOnQuestion(
 ) {
 	const user = requireAuth(db, args.token);
 	const id = newId();
+	const now = Date.now();
 	db.prepare(
-		`INSERT INTO comments (id, content, authorId, authorName, questionId, parentCommentId, createdAt)
-     VALUES (?, ?, ?, ?, ?, ?, ?)`,
+		`INSERT INTO comments (id, content, authorId, authorName, questionId, parentCommentId, createdAt, updatedAt)
+     VALUES (?, ?, ?, ?, ?, ?, ?, ?)`,
 	).run(
 		id,
 		args.content,
@@ -569,7 +572,8 @@ function commentsCreateOnQuestion(
 		user.name,
 		args.questionId,
 		args.parentCommentId ?? null,
-		Date.now(),
+		now,
+		now,
 	);
 	db.prepare("UPDATE questions SET answerCount = answerCount + 1 WHERE id = ?").run(
 		args.questionId,
@@ -599,7 +603,49 @@ function commentsRemove(db: Db, args: { token: string; id: string }) {
 		| { id: string; authorId: string }
 		| undefined;
 	if (!comment || comment.authorId !== user._id) throw new Error("Not authorized");
-	db.prepare("DELETE FROM comments WHERE id = ?").run(args.id);
+
+	const ids = [args.id];
+	const stack = [args.id];
+	while (stack.length > 0) {
+		const current = stack.pop()!;
+		const children = db
+			.prepare("SELECT id FROM comments WHERE parentCommentId = ?")
+			.all(current) as { id: string }[];
+		for (const child of children) {
+			ids.push(child.id);
+			stack.push(child.id);
+		}
+	}
+
+	db.exec("BEGIN");
+	try {
+		for (const id of ids) {
+			db.prepare("DELETE FROM comments WHERE id = ?").run(id);
+		}
+		db.exec("COMMIT");
+	} catch (err) {
+		db.exec("ROLLBACK");
+		throw err;
+	}
+}
+
+function commentsUpdate(db: Db, args: { token: string; id: string; content: string }) {
+	const user = requireAuth(db, args.token);
+	const comment = db.prepare("SELECT id, authorId FROM comments WHERE id = ?").get(args.id) as
+		| { id: string; authorId: string }
+		| undefined;
+	if (!comment || comment.authorId !== user._id) throw new Error("Not authorized");
+
+	const content = (args.content ?? "").trim();
+	if (!content) throw new Error("Comment cannot be empty");
+
+	const updatedAt = Date.now();
+	db.prepare("UPDATE comments SET content = ?, updatedAt = ? WHERE id = ?").run(
+		content,
+		updatedAt,
+		args.id,
+	);
+	return { updatedAt };
 }
 
 // ---- votes ----
@@ -1058,6 +1104,7 @@ const handlers: Record<string, Handler> = {
 	"comments:listByNote": commentsListByNote,
 	"comments:listByQuestion": commentsListByQuestion,
 	"comments:remove": commentsRemove,
+	"comments:update": commentsUpdate,
 	"votes:cast": votesCast,
 	"votes:getMyVote": votesGetMyVote,
 	"details:getNoteWithDetails": getNoteWithDetails,
diff --git a/src/lib/server/db.ts b/src/lib/server/db.ts
index ef310ac..a62706c 100644
--- a/src/lib/server/db.ts
+++ b/src/lib/server/db.ts
@@ -72,7 +72,8 @@ function createSchema(database: DatabaseSync) {
       parentId TEXT,
       questionId TEXT,
       parentCommentId TEXT,
-      createdAt INTEGER NOT NULL
+      createdAt INTEGER NOT NULL,
+      updatedAt INTEGER NOT NULL DEFAULT 0
     );
 
     CREATE TABLE IF NOT EXISTS votes (
@@ -220,6 +221,13 @@ function migrate(database: DatabaseSync) {
 	if (!unitColumns.some((c) => c.name === "code2")) {
 		database.exec("ALTER TABLE units ADD COLUMN code2 TEXT");
 	}
+
+	const commentColumns = database.prepare("PRAGMA table_info(comments)").all() as {
+		name: string;
+	}[];
+	if (!commentColumns.some((c) => c.name === "updatedAt")) {
+		database.exec("ALTER TABLE comments ADD COLUMN updatedAt INTEGER NOT NULL DEFAULT 0");
+	}
 }
 
 function seed(database: DatabaseSync) {
diff --git a/src/lib/types.ts b/src/lib/types.ts
index 4e3a676..24f3809 100644
--- a/src/lib/types.ts
+++ b/src/lib/types.ts
@@ -62,4 +62,5 @@ export type CommentDoc = Doc<"comments"> & {
 	questionId?: Id<"questions">;
 	parentCommentId?: Id<"comments">;
 	createdAt: number;
+	updatedAt?: number;
 };
diff --git a/src/routes/layout.css b/src/routes/layout.css
index e901f97..c33fd8b 100644
--- a/src/routes/layout.css
+++ b/src/routes/layout.css
@@ -124,6 +124,10 @@ button {
 	@apply text-ink hover:border-primary hover:text-primary bg-surface inline-flex h-8 min-w-8 items-center justify-center rounded-sm border border-transparent px-1.5 font-sans text-xs font-medium transition-colors;
 }
 
+.thread-branch {
+	@apply w-3 shrink-0 cursor-pointer self-stretch border-l-2 border-rule transition hover:border-secondary hover:brightness-150;
+}
+
 .markdown {
 	font-size: inherit;
 	line-height: inherit;
diff --git a/src/routes/notes/[id]/+page.svelte b/src/routes/notes/[id]/+page.svelte
index b6244fe..91e3515 100644
--- a/src/routes/notes/[id]/+page.svelte
+++ b/src/routes/notes/[id]/+page.svelte
@@ -7,6 +7,7 @@
 	import VoteStack from "$lib/components/VoteStack.svelte";
 	import Markdown from "$lib/components/Markdown.svelte";
 	import MarkdownEditor from "$lib/components/MarkdownEditor.svelte";
+	import CommentThread from "$lib/components/CommentThread.svelte";
 	import { timeAgo } from "$lib/time";
 	import type { NoteDoc, CommentDoc, TopicDoc, UnitDoc } from "$lib/types";
 
@@ -42,6 +43,11 @@
 		loading = false;
 	});
 
+	async function reloadComments() {
+		const updated = await query("details:getNoteWithDetails", { id: page.params.id });
+		if (updated) comments = (updated as any).comments ?? [];
+	}
+
 	async function postComment() {
 		if (!commentText.trim()) return;
 		const token = getToken();
@@ -59,8 +65,7 @@
 				parentId: page.params.id,
 			});
 			commentText = "";
-			const updated = await query("details:getNoteWithDetails", { id: page.params.id });
-			if (updated) comments = (updated as any).comments ?? [];
+			await reloadComments();
 		} catch (err: any) {
 			commentError = err.message ?? "Failed to post comment";
 		} finally {
@@ -250,36 +255,12 @@
 			{/if}
 
 			<div>
-				{#each comments as comment}
-					<div class="border-rule border-t py-4">
-						<p class="kicker">{comment.authorName} · {timeAgo(comment.createdAt)}</p>
-						<p class="text-ink mt-2 text-sm leading-relaxed whitespace-pre-wrap">
-							{comment.content}
-						</p>
-						{#if get(currentUser)?._id === comment.authorId}
-							<button
-								onclick={async () => {
-									const token = getToken();
-									if (!token) return;
-									try {
-										await mutation("comments:remove", {
-											token,
-											id: comment._id,
-										});
-										const updated = await query("details:getNoteWithDetails", {
-											id: page.params.id,
-										});
-										if (updated) comments = (updated as any).comments ?? [];
-									} catch (e) {}
-								}}
-								class="text-primary hover:text-primary-dark mt-2 text-xs"
-								>Delete</button
-							>
-						{/if}
-					</div>
-				{:else}
-					<p class="text-muted text-sm">No comments yet.</p>
-				{/each}
+				<CommentThread
+					{comments}
+					targetType="note"
+					targetId={note._id}
+					reload={reloadComments}
+				/>
 			</div>
 		</section>
 	{:else}
diff --git a/src/routes/questions/[id]/+page.svelte b/src/routes/questions/[id]/+page.svelte
index d17441a..b5de7c6 100644
--- a/src/routes/questions/[id]/+page.svelte
+++ b/src/routes/questions/[id]/+page.svelte
@@ -7,6 +7,7 @@
 	import VoteStack from "$lib/components/VoteStack.svelte";
 	import Markdown from "$lib/components/Markdown.svelte";
 	import MarkdownEditor from "$lib/components/MarkdownEditor.svelte";
+	import CommentThread from "$lib/components/CommentThread.svelte";
 	import { timeAgo } from "$lib/time";
 	import type { QuestionDoc, CommentDoc, TopicDoc, UnitDoc } from "$lib/types";
 
@@ -41,6 +42,11 @@
 		loading = false;
 	});
 
+	async function reloadAnswers() {
+		const updated = await query("details:getQuestionWithDetails", { id: page.params.id });
+		if (updated) answers = (updated as any).answers ?? [];
+	}
+
 	async function postAnswer() {
 		if (!answerText.trim()) return;
 		const token = getToken();
@@ -58,11 +64,7 @@
 				questionId: page.params.id,
 			});
 			answerText = "";
-			const updated = await query("details:getQuestionWithDetails", { id: page.params.id });
-			if (updated) {
-				question = { ...(updated as any), answers: undefined } as QuestionDoc;
-				answers = (updated as any).answers ?? [];
-			}
+			await reloadAnswers();
 		} catch (err: any) {
 			answerError = err.message ?? "Failed to post answer";
 		} finally {
@@ -250,37 +252,12 @@
 			{/if}
 
 			<div>
-				{#each answers as answer}
-					<div class="border-rule border-t py-4">
-						<p class="kicker">{answer.authorName} · {timeAgo(answer.createdAt)}</p>
-						<p class="text-ink mt-2 text-sm leading-relaxed whitespace-pre-wrap">
-							{answer.content}
-						</p>
-						{#if get(currentUser)?._id === answer.authorId}
-							<button
-								onclick={async () => {
-									const token = getToken();
-									if (!token) return;
-									try {
-										await mutation("comments:remove", {
-											token,
-											id: answer._id,
-										});
-										const updated = await query(
-											"details:getQuestionWithDetails",
-											{ id: page.params.id },
-										);
-										if (updated) answers = (updated as any).answers ?? [];
-									} catch (e) {}
-								}}
-								class="text-primary hover:text-primary-dark mt-2 text-xs"
-								>Delete</button
-							>
-						{/if}
-					</div>
-				{:else}
-					<p class="text-muted text-sm">No answers yet.</p>
-				{/each}
+				<CommentThread
+					comments={answers}
+					targetType="question"
+					targetId={question._id}
+					reload={reloadAnswers}
+				/>
 			</div>
 		</section>
 	{:else}