diff --git a/sub_crates/backend/src/marks.rs b/sub_crates/backend/src/marks.rs
index a6a9b1a..c80f8ad 100644
--- a/sub_crates/backend/src/marks.rs
+++ b/sub_crates/backend/src/marks.rs
@@ -128,6 +128,10 @@ impl MarkSet {
         }
     }
 
+    pub fn len(&self) -> usize {
+        self.marks.len()
+    }
+
     pub fn clear(&mut self) {
         self.main_mark_idx = 0;
         self.marks.clear();
diff --git a/sub_crates/backend/src/transaction.rs b/sub_crates/backend/src/transaction.rs
index a5066ec..4694156 100644
--- a/sub_crates/backend/src/transaction.rs
+++ b/sub_crates/backend/src/transaction.rs
@@ -359,8 +359,52 @@ impl Transaction {
     }
 
     /// Applies the Transaction to a set of Marks.
-    pub fn apply_to_marks(&self, _marks: &mut MarkSet) {
-        todo!()
+    pub fn apply_to_marks(&self, marks: &mut MarkSet) {
+        marks.make_consistent();
+
+        // We do this in two passes: first heads, then tails.
+        for tail in 0..=1 {
+            let mut mark_idx = 0;
+            let mut byte_idx_old = 0;
+            let mut byte_idx_new = 0;
+            for op in self.ops.iter() {
+                let old_len = op.len_pre();
+                let new_len = op.len_post();
+
+                while mark_idx < marks.len() {
+                    let idx = if tail == 1 {
+                        &mut marks[mark_idx].tail
+                    } else {
+                        &mut marks[mark_idx].head
+                    };
+
+                    if *idx < byte_idx_old {
+                        mark_idx += 1;
+                    } else if *idx < (byte_idx_old + old_len) {
+                        *idx = *idx + byte_idx_new - byte_idx_old;
+                        *idx = (*idx).min(byte_idx_new + new_len);
+                        mark_idx += 1;
+                    } else {
+                        break;
+                    }
+                }
+
+                byte_idx_old += old_len;
+                byte_idx_new += new_len;
+            }
+
+            while mark_idx < marks.len() {
+                let idx = if tail == 1 {
+                    &mut marks[mark_idx].tail
+                } else {
+                    &mut marks[mark_idx].head
+                };
+                *idx = *idx + byte_idx_new - byte_idx_old;
+                mark_idx += 1;
+            }
+        }
+
+        marks.make_consistent();
     }
 
     /// Applies the inverse of the Transaction to a set of Marks.
@@ -368,8 +412,52 @@ impl Transaction {
     /// This is essentially an "undo" of the Transaction.  However, on
     /// marks this is a lossy operation and is not guaranteed to return
     /// the marks to their original count and position.
-    pub fn apply_inverse_to_marks(&self, _marks: &mut MarkSet) {
-        todo!()
+    pub fn apply_inverse_to_marks(&self, marks: &mut MarkSet) {
+        marks.make_consistent();
+
+        // We do this in two passes: first heads, then tails.
+        for tail in 0..=1 {
+            let mut mark_idx = 0;
+            let mut byte_idx_old = 0;
+            let mut byte_idx_new = 0;
+            for op in self.ops.iter() {
+                let old_len = op.len_post();
+                let new_len = op.len_pre();
+
+                while mark_idx < marks.len() {
+                    let idx = if tail == 1 {
+                        &mut marks[mark_idx].tail
+                    } else {
+                        &mut marks[mark_idx].head
+                    };
+
+                    if *idx < byte_idx_old {
+                        mark_idx += 1;
+                    } else if *idx < (byte_idx_old + old_len) {
+                        *idx = *idx + byte_idx_new - byte_idx_old;
+                        *idx = (*idx).min(byte_idx_new + new_len);
+                        mark_idx += 1;
+                    } else {
+                        break;
+                    }
+                }
+
+                byte_idx_old += old_len;
+                byte_idx_new += new_len;
+            }
+
+            while mark_idx < marks.len() {
+                let idx = if tail == 1 {
+                    &mut marks[mark_idx].tail
+                } else {
+                    &mut marks[mark_idx].head
+                };
+                *idx = *idx + byte_idx_new - byte_idx_old;
+                mark_idx += 1;
+            }
+        }
+
+        marks.make_consistent();
     }
 
     //---------------------------------------------------------
@@ -455,3 +543,143 @@ impl std::fmt::Debug for Transaction {
         Ok(())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::marks::{Mark, MarkSet};
+
+    #[test]
+    fn apply_to_marks_01() {
+        let trans = Transaction::from_edit(5, "", "hi");
+
+        let mut mark_set = MarkSet::new();
+        mark_set.add_mark(Mark::new(4, 4));
+        mark_set.add_mark(Mark::new(5, 5));
+        mark_set.add_mark(Mark::new(6, 6));
+
+        trans.apply_to_marks(&mut mark_set);
+        assert_eq!(mark_set.marks[0].head, 4);
+        assert_eq!(mark_set.marks[0].tail, 4);
+        assert_eq!(mark_set.marks[1].head, 7);
+        assert_eq!(mark_set.marks[1].tail, 7);
+        assert_eq!(mark_set.marks[2].head, 8);
+        assert_eq!(mark_set.marks[2].tail, 8);
+    }
+
+    #[test]
+    fn apply_to_marks_02() {
+        let trans = Transaction {
+            ops: vec![
+                Op::Retain(5),
+                Op::Replace {
+                    old: (0, 0),
+                    new: (0, 1),
+                },
+                Op::Replace {
+                    old: (0, 0),
+                    new: (1, 2),
+                },
+            ],
+            buffer: "hi".into(),
+        };
+
+        let mut mark_set = MarkSet::new();
+        mark_set.add_mark(Mark::new(4, 4));
+        mark_set.add_mark(Mark::new(5, 5));
+        mark_set.add_mark(Mark::new(6, 6));
+
+        trans.apply_to_marks(&mut mark_set);
+        assert_eq!(mark_set.marks[0].head, 4);
+        assert_eq!(mark_set.marks[0].tail, 4);
+        assert_eq!(mark_set.marks[1].head, 7);
+        assert_eq!(mark_set.marks[1].tail, 7);
+        assert_eq!(mark_set.marks[2].head, 8);
+        assert_eq!(mark_set.marks[2].tail, 8);
+    }
+
+    #[test]
+    fn apply_to_marks_03() {
+        let trans = Transaction::from_edit(5, "hi", "");
+
+        let mut mark_set = MarkSet::new();
+        mark_set.add_mark(Mark::new(4, 4));
+        mark_set.add_mark(Mark::new(6, 6));
+        mark_set.add_mark(Mark::new(8, 8));
+
+        trans.apply_to_marks(&mut mark_set);
+        assert_eq!(mark_set.marks[0].head, 4);
+        assert_eq!(mark_set.marks[0].tail, 4);
+        assert_eq!(mark_set.marks[1].head, 5);
+        assert_eq!(mark_set.marks[1].tail, 5);
+        assert_eq!(mark_set.marks[2].head, 6);
+        assert_eq!(mark_set.marks[2].tail, 6);
+    }
+
+    #[test]
+    fn apply_to_marks_04() {
+        let trans = Transaction {
+            ops: vec![
+                Op::Retain(5),
+                Op::Replace {
+                    old: (0, 1),
+                    new: (0, 0),
+                },
+                Op::Replace {
+                    old: (1, 2),
+                    new: (0, 0),
+                },
+            ],
+            buffer: "hi".into(),
+        };
+
+        let mut mark_set = MarkSet::new();
+        mark_set.add_mark(Mark::new(4, 4));
+        mark_set.add_mark(Mark::new(6, 6));
+        mark_set.add_mark(Mark::new(8, 8));
+
+        trans.apply_to_marks(&mut mark_set);
+        assert_eq!(mark_set.marks[0].head, 4);
+        assert_eq!(mark_set.marks[0].tail, 4);
+        assert_eq!(mark_set.marks[1].head, 5);
+        assert_eq!(mark_set.marks[1].tail, 5);
+        assert_eq!(mark_set.marks[2].head, 6);
+        assert_eq!(mark_set.marks[2].tail, 6);
+    }
+
+    #[test]
+    fn apply_inverse_to_marks_01() {
+        let trans = Transaction::from_edit(5, "hi", "");
+
+        let mut mark_set = MarkSet::new();
+        mark_set.add_mark(Mark::new(4, 4));
+        mark_set.add_mark(Mark::new(5, 5));
+        mark_set.add_mark(Mark::new(6, 6));
+
+        trans.apply_inverse_to_marks(&mut mark_set);
+        assert_eq!(mark_set.marks[0].head, 4);
+        assert_eq!(mark_set.marks[0].tail, 4);
+        assert_eq!(mark_set.marks[1].head, 7);
+        assert_eq!(mark_set.marks[1].tail, 7);
+        assert_eq!(mark_set.marks[2].head, 8);
+        assert_eq!(mark_set.marks[2].tail, 8);
+    }
+
+    #[test]
+    fn apply_inverse_to_marks_02() {
+        let trans = Transaction::from_edit(5, "", "hi");
+
+        let mut mark_set = MarkSet::new();
+        mark_set.add_mark(Mark::new(4, 4));
+        mark_set.add_mark(Mark::new(6, 6));
+        mark_set.add_mark(Mark::new(8, 8));
+
+        trans.apply_inverse_to_marks(&mut mark_set);
+        assert_eq!(mark_set.marks[0].head, 4);
+        assert_eq!(mark_set.marks[0].tail, 4);
+        assert_eq!(mark_set.marks[1].head, 5);
+        assert_eq!(mark_set.marks[1].tail, 5);
+        assert_eq!(mark_set.marks[2].head, 6);
+        assert_eq!(mark_set.marks[2].tail, 6);
+    }
+}