diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1430108..ffd0c0a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -28,6 +28,8 @@ add_executable(ke
         core.c
         core.h
         main.c
+        undo.h
+        undo.c
 )
 target_compile_definitions(ke PRIVATE KE_VERSION="ke version ${KE_VERSION}")
 install(TARGETS ke RUNTIME DESTINATION bin)
diff --git a/abuf.c b/abuf.c
index a1f0dd5..3732b25 100644
--- a/abuf.c
+++ b/abuf.c
@@ -1,6 +1,7 @@
 #include <assert.h>
 #include <stdlib.h>
 #include <string.h>
+#include <stdint-gcc.h>
 
 #include "abuf.h"
 #include "core.h"
@@ -36,6 +37,16 @@ ab_init_cap(abuf *buf, const size_t cap)
 }
 
 
+void
+ab_init_str(abuf *buf, const char *s)
+{
+	size_t	 len = kstrnlen(s, SIZE_MAX);
+
+	ab_init_cap(buf, len);
+	ab_append(buf, s, len);
+}
+
+
 void
 ab_resize(abuf *buf, size_t cap)
 {
@@ -71,6 +82,18 @@ ab_append(abuf *buf, const char *s, size_t len)
 }
 
 
+void
+ab_append_ab(abuf *buf, abuf *other)
+{
+	assert(buf != NULL && other != NULL);
+	if (other->size == 0) {
+		return;
+	}
+
+	ab_append(buf, other->b, other->size);
+}
+
+
 void
 ab_prependch(abuf *buf, const char c)
 {
@@ -97,6 +120,18 @@ ab_prepend(abuf *buf, const char *s, const size_t len)
 }
 
 
+void
+ab_prepend_ab(abuf *buf, abuf *other)
+{
+	assert(buf != NULL && other != NULL);
+	if (other->size == 0) {
+		return;
+	}
+
+	ab_prepend(buf, other->b, other->size);
+}
+
+
 void
 ab_free(abuf *buf)
 {
diff --git a/abuf.h b/abuf.h
index 90d87f3..b793c92 100644
--- a/abuf.h
+++ b/abuf.h
@@ -19,11 +19,14 @@ typedef struct abuf {
 
 void		 ab_init(abuf *buf);
 void		 ab_init_cap(abuf *buf, size_t cap);
+void		 ab_init_str(abuf *buf, const char *s);
 void		 ab_resize(abuf *buf, size_t cap);
 void		 ab_appendch(abuf *buf, char c);
 void		 ab_append(abuf *buf, const char *s, size_t len);
-void		 ab_prependch(abuf *buf, const char c);
-void		 ab_prepend(abuf *buf, const char *s, const size_t len);
+void		 ab_append_ab(abuf *buf, abuf *other);
+void		 ab_prependch(abuf *buf, char c);
+void		 ab_prepend(abuf *buf, const char *s, size_t len);
+void		 ab_prepend_ab(abuf *buf, abuf *other);
 void		 ab_free(abuf *buf);
 
 
diff --git a/buffer.c b/buffer.c
index 7f17f14..e5226ba 100644
--- a/buffer.c
+++ b/buffer.c
@@ -271,7 +271,9 @@ buffer_add_empty(void)
 	buf->dirty     = 0;
 	buf->mark_set  = 0;
 	buf->mark_curx = 0;
-	buf->mark_cury = 0;
+ buf->mark_cury = 0;
+ /* initialize undo tree for this buffer */
+ undo_tree_init(&buf->undo);
 
     editor.buffers[editor.bufcount] = buf;
     idx                             = (int)editor.bufcount;
@@ -397,14 +399,16 @@ buffer_close_current(void)
 		buffer_switch(nb);
 	}
 
-	b = editor.buffers[closing];
-	if (b) {
-		if (b->row) {
-			for (size_t i = 0; i < b->nrows; i++) {
-				ab_free(&b->row[i]);
-			}
-			free(b->row);
-		}
+ b = editor.buffers[closing];
+ if (b) {
+     /* free undo tree resources before freeing buffer */
+     undo_tree_free(&b->undo);
+     if (b->row) {
+         for (size_t i = 0; i < b->nrows; i++) {
+             ab_free(&b->row[i]);
+         }
+         free(b->row);
+     }
 
 		if (b->filename) {
 			free(b->filename);
diff --git a/buffer.h b/buffer.h
index 1289801..d6f57d3 100644
--- a/buffer.h
+++ b/buffer.h
@@ -2,18 +2,20 @@
 #define KE_BUFFER_H
 
 #include "abuf.h"
+#include "undo.h"
 
 
 typedef struct buffer {
-	size_t	 curx, cury;
-	size_t	 rx;
-	size_t	 nrows;
-	size_t	 rowoffs, coloffs;
-	abuf	*row;
-	char	*filename;
-	int	 dirty;
-	int	 mark_set;
-	size_t	 mark_curx, mark_cury;
+	size_t		 curx, cury;
+	size_t		 rx;
+	size_t		 nrows;
+	size_t		 rowoffs, coloffs;
+	abuf		*row;
+	char		*filename;
+	int		 dirty;
+	int		 mark_set;
+	size_t		 mark_curx, mark_cury;
+	undo_tree	 undo;
 } buffer;
 
 /* Access current buffer and convenient aliases for file-specific fields */
diff --git a/ke.1 b/ke.1
index 38774ac..183bd76 100644
--- a/ke.1
+++ b/ke.1
@@ -65,9 +65,9 @@ Reload the current buffer from disk.
 .It C-k s
 Save the file, prompting for a filename if needed. Also C-k C-s.
 .It C-k u
-Undo changes (not implemented; marking this k-command as taken).
+Undo changes.
 .It C-k U
-Redo changes (not implemented; marking this k-command as taken).
+Redo changes.
 .It C-k x
 save the file and exit. Also C-k C-x.
 .It C-k y
diff --git a/main.c b/main.c
index f101cf1..1f1af8c 100644
--- a/main.c
+++ b/main.c
@@ -1121,18 +1121,27 @@ insertch(const int16_t c)
 	 * a row; it can just figure out where the cursor is
 	 * at and what to do.
 	 */
-	if (ECURY == ENROWS) {
-		erow_insert(ENROWS, "", 0);
-	}
+ if (ECURY == ENROWS) {
+        erow_insert(ENROWS, "", 0);
+    }
 
-	/* Inserting ends kill ring chaining. */
-	editor.kill = 0;
+    /* Inserting ends kill ring chaining. */
+    editor.kill = 0;
 
-	row_insert_ch(&EROW[ECURY],
-		      ECURX,
-		      (int16_t) (c & 0xff));
-	ECURX++;
-	EDIRTY++;
+    /* Begin/append undo record for insert operations */
+    undo_tree *utree = &CURBUF->undo;
+    undo_begin(utree, UNDO_INSERT);
+    if (utree->pending && utree->pending->text.size == 0) {
+        utree->pending->row = ECURY;
+        utree->pending->col = ECURX;
+    }
+    undo_appendch(utree, (char)(c & 0xff));
+
+    row_insert_ch(&EROW[ECURY],
+                  ECURX,
+                  (int16_t) (c & 0xff));
+    ECURX++;
+    EDIRTY++;
 }
 
 
@@ -1905,15 +1914,24 @@ move_cursor(const int16_t c, const int interactive)
 void
 newline(void)
 {
-	abuf	*row     = NULL;
-	size_t	 rhs_len = 0;
-	char	*tmp     = NULL;
+    abuf	*row     = NULL;
+    size_t	 rhs_len = 0;
+    char	*tmp     = NULL;
 
-	if (ECURY >= ENROWS) {
-		erow_insert(ECURY, "", 0);
-		ECURY++;
-		ECURX = 0;
-	} else if (ECURX == 0) {
+    /* Begin/append undo record for insert operations (newline as '\n') */
+    undo_tree *utree = &CURBUF->undo;
+    undo_begin(utree, UNDO_INSERT);
+    if (utree->pending && utree->pending->text.size == 0) {
+        utree->pending->row = ECURY;
+        utree->pending->col = ECURX;
+    }
+    undo_appendch(utree, '\n');
+
+    if (ECURY >= ENROWS) {
+        erow_insert(ECURY, "", 0);
+        ECURY++;
+        ECURX = 0;
+    } else if (ECURX == 0) {
 		erow_insert(ECURY, "", 0);
 		ECURY++;
 		ECURX = 0;
@@ -2177,14 +2195,16 @@ process_kcommand(const int16_t c)
 		case 'u':
 			reps = uarg_get();
 
-			while (reps--) {}
-			editor_set_status("Undo not implemented.");
+			while (reps--) {
+				editor_undo(CURBUF);
+			}
 			break;
 		case 'U':
 			reps = uarg_get();
 
-			while (reps--) {}
-			editor_set_status("Redo not implemented.");
+			while (reps--) {
+				editor_redo(CURBUF);
+			}
 			break;
 		case 'y':
 			reps = uarg_get();
diff --git a/undo.c b/undo.c
index 50551c8..5865b7b 100644
--- a/undo.c
+++ b/undo.c
@@ -1,6 +1,9 @@
 #include <assert.h>
 #include <stdlib.h>
+#include <string.h>
 #include "abuf.h"
+#include "editor.h"
+#include "buffer.h"
 #include "undo.h"
 
 
@@ -27,8 +30,6 @@ undo_node_new(undo_kind kind)
 void
 undo_node_free(undo_node *node)
 {
-	undo_node	*next = NULL;
-
 	if (node == NULL) {
 		return;
 	}
@@ -100,15 +101,276 @@ undo_begin(undo_tree *tree, undo_kind kind)
 void
 undo_prepend(undo_tree *tree, abuf *buf)
 {
+	assert(tree != NULL);
+	assert(tree->pending != NULL);
 
+	ab_prepend_ab(&tree->pending->text, buf);
 }
 
 
-void		 undo_append(undo_tree *tree, abuf *buf);
-void		 undo_prependch(undo_tree *tree, char c);
-void		 undo_appendch(undo_tree *tree, char c);
-void		 undo_commit(undo_tree *tree);
-void		 undo_apply(struct editor *editor);
-void		 editor_undo(undo_tree *tree);
-void		 editor_redo(undo_tree *tree);
+void
+undo_append(undo_tree *tree, abuf *buf)
+{
+	assert(tree != NULL);
+	assert(tree->pending != NULL);
+
+	ab_append_ab(&tree->pending->text, buf);
+}
+
+
+void
+undo_prependch(undo_tree *tree, char c)
+{
+	assert(tree != NULL);
+	assert(tree->pending != NULL);
+
+	ab_prependch(&tree->pending->text, c);
+}
+
+
+void
+undo_appendch(undo_tree *tree, char c)
+{
+	assert(tree != NULL);
+	assert(tree->pending != NULL);
+
+	ab_appendch(&tree->pending->text, c);
+}
+
+
+void
+undo_commit(undo_tree *tree)
+{
+	assert(tree != NULL);
+
+	if (tree->pending == NULL) {
+		return;
+	}
+
+	if (tree->root == NULL) {
+		tree->root = tree->pending;
+		tree->current = tree->pending;
+		tree->pending = NULL;
+		return;
+	}
+
+	undo_node_free_all(tree->current->next);
+	tree->current->next = tree->pending;
+	tree->pending->parent = tree->current;
+	tree->current = tree->pending;
+	tree->pending = NULL;
+}
+
+
+/* --- Helper functions for applying undo/redo operations --- */
+static void
+row_insert_at(buffer *b, size_t at, const char *s, size_t len)
+{
+    abuf *newrows = realloc(b->row, sizeof(abuf) * (b->nrows + 1));
+    assert(newrows != NULL);
+    b->row = newrows;
+
+    if (at < b->nrows) {
+        memmove(&b->row[at + 1], &b->row[at], sizeof(abuf) * (b->nrows - at));
+    }
+    ab_init(&b->row[at]);
+    if (len > 0) {
+        ab_append(&b->row[at], s, len);
+    }
+    b->nrows++;
+    b->dirty++;
+}
+
+static void
+ensure_row_exists(buffer *b, size_t r)
+{
+    while (r >= b->nrows) {
+        row_insert_at(b, b->nrows, "", 0);
+    }
+}
+
+static void
+row_insert_ch_at(abuf *row, size_t col, char ch)
+{
+    if (col > row->size) {
+        col = row->size;
+    }
+    ab_resize(row, row->size + 2);
+    memmove(&row->b[col + 1], &row->b[col], row->size - col + 1);
+    row->b[col] = ch;
+    row->size++;
+    row->b[row->size] = '\0';
+}
+
+static void
+row_delete_ch_at(abuf *row, size_t col)
+{
+    if (col >= row->size) {
+        return;
+    }
+    memmove(&row->b[col], &row->b[col + 1], row->size - col);
+    row->size--;
+    row->b[row->size] = '\0';
+}
+
+static void
+split_row_at(buffer *b, size_t r, size_t c)
+{
+    ensure_row_exists(b, r);
+    abuf *row = &b->row[r];
+    if (c > row->size) c = row->size;
+    size_t rhs_len = row->size - c;
+    char *rhs = NULL;
+    if (rhs_len > 0) {
+        rhs = malloc(rhs_len);
+        assert(rhs != NULL);
+        memcpy(rhs, &row->b[c], rhs_len);
+    }
+    row->size = c;
+    if (row->cap <= row->size) {
+        ab_resize(row, row->size + 1);
+    }
+    row->b[row->size] = '\0';
+
+    row_insert_at(b, r + 1, rhs ? rhs : "", rhs_len);
+    if (rhs) free(rhs);
+    b->dirty++;
+}
+
+static void
+join_with_next_row(buffer *b, size_t r, size_t c)
+{
+    if (r >= b->nrows) return;
+    abuf *row = &b->row[r];
+    if (c > row->size) c = row->size;
+    if (r + 1 >= b->nrows) return;
+    abuf *next = &b->row[r + 1];
+
+    /* Make room in current row at end if needed (we append next->b after position c). */
+    size_t tail_len = row->size - c;
+    size_t add_len = next->size;
+    ab_resize(row, row->size + add_len + 1);
+    /* Move tail to make room for next content */
+    memmove(&row->b[c + add_len], &row->b[c], tail_len);
+    memcpy(&row->b[c], next->b, add_len);
+    row->size += add_len;
+    row->b[row->size] = '\0';
+
+    /* Delete next row */
+    ab_free(next);
+    if (b->nrows - (r + 2) > 0) {
+        memmove(&b->row[r + 1], &b->row[r + 2], sizeof(abuf) * (b->nrows - (r + 2)));
+    }
+    b->nrows--;
+    b->dirty++;
+}
+
+static void
+buffer_insert_text(buffer *b, size_t row, size_t col, const char *s, size_t len)
+{
+    ensure_row_exists(b, row);
+    size_t r = row;
+    size_t c = col;
+    if (c > b->row[r].size) c = b->row[r].size;
+    for (size_t i = 0; i < len; i++) {
+        char ch = s[i];
+        if (ch == '\n') {
+            split_row_at(b, r, c);
+            r++;
+            c = 0;
+        } else {
+            row_insert_ch_at(&b->row[r], c, ch);
+            c++;
+        }
+        b->dirty++;
+    }
+}
+
+static void
+buffer_delete_text(buffer *b, size_t row, size_t col, const char *s, size_t len)
+{
+    if (row >= b->nrows) return;
+    size_t r = row;
+    size_t c = col;
+    if (c > b->row[r].size) c = b->row[r].size;
+    for (size_t i = 0; i < len; i++) {
+        char ch = s[i];
+        if (ch == '\n') {
+            /* delete newline at (r,c): join row r with next */
+            join_with_next_row(b, r, c);
+        } else {
+            row_delete_ch_at(&b->row[r], c);
+            /* c stays the same because character at c is removed */
+        }
+        b->dirty++;
+        if (r >= b->nrows) break; /* safety */
+        if (c > b->row[r].size) c = b->row[r].size;
+    }
+}
+
+void
+undo_apply(struct buffer *buf, int direction) {
+	undo_tree	*tree = &buf->undo;
+	undo_node	*node = NULL;
+	undo_commit(tree);
+
+	if (tree->root == NULL) {
+		return;
+	}
+
+	if (direction < 0) {
+		/* UNDO: apply inverse of current node, then move back */
+		node = tree->current;
+		if (node == NULL) {
+			return;
+		}
+		switch (node->kind) {
+			/* support insert first */
+			case UNDO_INSERT:
+				buffer_delete_text(buf, node->row, node->col,
+						   node->text.b, node->text.size);
+				break;
+			default:
+				/* unknown type: do nothing */
+				break;
+		}
+
+		tree->current = node->parent; /* move back in history */
+	} else if (direction > 0) {
+		/* REDO: move forward then apply node */
+		undo_node *next = (tree->current == NULL)
+				  ? tree->root
+				  : tree->current->next;
+		if (next == NULL) {
+			return; /* nothing to redo */
+		}
+
+		switch (next->kind) {
+			/* support insert first */
+			case UNDO_INSERT:
+				buffer_insert_text(buf, next->row, next->col,
+						   next->text.b, next->text.size);
+				break;
+			default:
+				/* unknown type: do nothing */
+				break;
+		}
+
+		tree->current = next; /* move forward in history */
+	}
+}
+
+
+void
+editor_undo(struct buffer *buf)
+{
+	undo_apply(buf, -1);
+}
+
+
+void
+editor_redo(struct buffer *buf)
+{
+	undo_apply(buf, 1);
+}
 
diff --git a/undo.h b/undo.h
index f33e5f5..9d2f056 100644
--- a/undo.h
+++ b/undo.h
@@ -1,13 +1,16 @@
 #include <stddef.h>
 
 #include "abuf.h"
-#include "editor.h"
+#include "buffer.h"
 
 
 #ifndef KE_UNDO_H
 #define KE_UNDO_H
 
 
+struct buffer;
+
+
 typedef enum undo_kind {
 	UNDO_INSERT  = 1 << 0,
 	UNDO_UNKNOWN = 1 << 1,
@@ -42,9 +45,9 @@ void		 undo_append(undo_tree *tree, abuf *buf);
 void		 undo_prependch(undo_tree *tree, char c);
 void		 undo_appendch(undo_tree *tree, char c);
 void		 undo_commit(undo_tree *tree);
-void		 undo_apply(struct editor *editor);
-void		 editor_undo(undo_tree *tree);
-void		 editor_redo(undo_tree *tree);
+void		 undo_apply(struct buffer *buf, int direction);
+void		 editor_undo(struct buffer *buf);
+void		 editor_redo(struct buffer *buf);
 
 
 #endif