diff --git a/engines/types.ts b/engines/types.ts
index 0901c13..0972133 100644
--- a/engines/types.ts
+++ b/engines/types.ts
@@ -14,6 +14,14 @@ export type DocumentRange = {
   charRange?: CharRange;
 };
 
+/** Type denoting a document edit */
+export type DocumentEdit = {
+  /** Range to replace with the given text. Keep empty to insert text */
+  range: DocumentRange;
+  /** Text to replace the given range with */
+  text: string;
+};
+
 /** Source code token provider for the language, specific to Monaco */
 export type MonacoTokensProvider = monaco.languages.IMonarchLanguage;
 
@@ -30,6 +38,9 @@ export type StepExecutionResult<RS> = {
   /** String to write to program output */
   output?: string;
 
+  /** Self-modifying programs: edit to apply on code */
+  codeEdits?: DocumentEdit[];
+
   /**
    * Used to highlight next line to be executed in the editor.
    * Passing `null` indicates reaching the end of program.
diff --git a/ui/Mainframe.tsx b/ui/Mainframe.tsx
index 6f6102d..31681b4 100644
--- a/ui/Mainframe.tsx
+++ b/ui/Mainframe.tsx
@@ -44,6 +44,15 @@ export const Mainframe = <RS extends {}>({ langName, provider }: Props<RS>) => {
     rendererRef.current!.updateState(result.rendererState);
     codeEditorRef.current!.updateHighlights(result.nextStepLocation);
     outputEditorRef.current!.append(result.output);
+
+    // Self-modifying programs: update code
+    if (result.codeEdits != null)
+      codeEditorRef.current!.editCode(result.codeEdits);
+
+    // End of program: reset code to original version
+    if (!result.nextStepLocation) codeEditorRef.current!.endExecutionMode();
+
+    // RuntimeError: print error to output
     if (error) outputEditorRef.current!.setError(error);
   };
 
@@ -60,13 +69,16 @@ export const Mainframe = <RS extends {}>({ langName, provider }: Props<RS>) => {
     outputEditorRef.current!.reset();
     await execController.resetState();
     const error = await execController.prepare(
-      codeEditorRef.current!.getValue(),
+      codeEditorRef.current!.getCode(),
       inputEditorRef.current!.getValue()
     );
 
     // Check for ParseError, else begin execution
     if (error) outputEditorRef.current!.setError(error);
-    else await execController.execute(updateWithResult, execInterval);
+    else {
+      codeEditorRef.current!.startExecutionMode();
+      await execController.execute(updateWithResult, execInterval);
+    }
   };
 
   /** Pause the ongoing execution */
@@ -120,6 +132,7 @@ export const Mainframe = <RS extends {}>({ langName, provider }: Props<RS>) => {
     await execController.resetState();
     rendererRef.current!.updateState(null);
     codeEditorRef.current!.updateHighlights(null);
+    codeEditorRef.current!.endExecutionMode();
   };
 
   /** Translate execution controller state to debug controls state */
@@ -137,7 +150,6 @@ export const Mainframe = <RS extends {}>({ langName, provider }: Props<RS>) => {
         <CodeEditor
           ref={codeEditorRef}
           languageId={langName}
-          readOnly={execController.state === "processing"}
           defaultValue={providerRef.current.sampleProgram}
           tokensProvider={providerRef.current.editorTokensProvider}
           onValidateCode={execController.validateCode}
diff --git a/ui/code-editor/index.tsx b/ui/code-editor/index.tsx
index 41cfcdc..278e422 100644
--- a/ui/code-editor/index.tsx
+++ b/ui/code-editor/index.tsx
@@ -1,9 +1,14 @@
 import React from "react";
 import Editor from "@monaco-editor/react";
 import { useEditorLanguageConfig } from "./use-editor-lang-config";
-import { DocumentRange, MonacoTokensProvider } from "../../engines/types";
+import {
+  DocumentEdit,
+  DocumentRange,
+  MonacoTokensProvider,
+} from "../../engines/types";
 import {
   createHighlightRange,
+  createMonacoDocEdit,
   EditorInstance,
   MonacoInstance,
 } from "./monaco-utils";
@@ -14,12 +19,26 @@ import { useDarkMode } from "../providers/dark-mode-provider";
 import { WorkerParseError } from "../../engines/worker-errors";
 import { useCodeValidator } from "./use-code-validator";
 
+/** Keeps track of user's original program and modifications done to it */
+type ChangeTrackerValue = {
+  /** The user's original program code */
+  original: string;
+  /** Tracks if code was modified during execution */
+  changed: boolean;
+};
+
 // Interface for interacting with the editor
 export interface CodeEditorRef {
   /** Get the current text content of the editor */
-  getValue: () => string;
+  getCode: () => string;
+  /** Update value of code */
+  editCode: (edits: DocumentEdit[]) => void;
   /** Update code highlights */
   updateHighlights: (highlights: DocumentRange | null) => void;
+  /** Start execution mode - readonly editor with modifyable contents */
+  startExecutionMode: () => void;
+  /** End execution mode - reset contents and readonly state */
+  endExecutionMode: () => void;
 }
 
 type Props = {
@@ -29,8 +48,6 @@ type Props = {
   defaultValue: string;
   /** Tokens provider for the language */
   tokensProvider?: MonacoTokensProvider;
-  /** Set editor as read-only */
-  readOnly?: boolean;
   /** Callback to validate code syntax */
   onValidateCode: (code: string) => Promise<WorkerParseError | undefined>;
   /** Callback to update debugging breakpoints */
@@ -44,9 +61,16 @@ type Props = {
 const CodeEditorComponent = (props: Props, ref: React.Ref<CodeEditorRef>) => {
   const [editor, setEditor] = React.useState<EditorInstance | null>(null);
   const [monaco, setMonaco] = React.useState<MonacoInstance | null>(null);
+  const [readOnly, setReadOnly] = React.useState(false);
   const highlightRange = React.useRef<string[]>([]);
   const { isDark } = useDarkMode();
 
+  // Code modification tracker, used in execution mode
+  const changeTracker = React.useRef<ChangeTrackerValue>({
+    original: "",
+    changed: false,
+  });
+
   // Breakpoints
   useEditorBreakpoints({
     editor,
@@ -89,8 +113,25 @@ const CodeEditorComponent = (props: Props, ref: React.Ref<CodeEditorRef>) => {
   React.useImperativeHandle(
     ref,
     () => ({
-      getValue: () => editor!.getValue(),
+      getCode: () => editor!.getValue(),
+      editCode: (edits) => {
+        changeTracker.current.changed = true;
+        const monacoEdits = edits.map(createMonacoDocEdit);
+        editor!.getModel()!.applyEdits(monacoEdits);
+      },
       updateHighlights,
+      startExecutionMode: () => {
+        changeTracker.current.original = editor!.getValue();
+        changeTracker.current.changed = false;
+        setReadOnly(true);
+      },
+      endExecutionMode: () => {
+        setReadOnly(false);
+        if (changeTracker.current.changed) {
+          editor!.getModel()!.setValue(changeTracker.current.original);
+          changeTracker.current.changed = false;
+        }
+      },
     }),
     [editor]
   );
@@ -112,7 +153,7 @@ const CodeEditorComponent = (props: Props, ref: React.Ref<CodeEditorRef>) => {
       options={{
         minimap: { enabled: false },
         glyphMargin: true,
-        readOnly: props.readOnly,
+        readOnly: readOnly,
       }}
     />
   );
diff --git a/ui/code-editor/monaco-utils.ts b/ui/code-editor/monaco-utils.ts
index f2ce699..daebfce 100644
--- a/ui/code-editor/monaco-utils.ts
+++ b/ui/code-editor/monaco-utils.ts
@@ -1,5 +1,5 @@
 import monaco from "monaco-editor";
-import { DocumentRange } from "../../engines/types";
+import { DocumentEdit, DocumentRange } from "../../engines/types";
 import { WorkerParseError } from "../../engines/worker-errors";
 
 /** Type alias for an instance of Monaco editor */
@@ -60,6 +60,26 @@ export const createValidationMarker = (
   };
 };
 
+/**
+ * Convert a DocumentEdit instance to Monaco edit object format.
+ * @param edit DocumentEdit to convert to Monaco format
+ * @returns Instance of Monaco's edit object
+ */
+export const createMonacoDocEdit = (
+  edit: DocumentEdit
+): monaco.editor.IIdentifiedSingleEditOperation => {
+  const location = get1IndexedLocation(edit.range);
+  return {
+    text: edit.text,
+    range: {
+      startLineNumber: location.line,
+      endLineNumber: location.line,
+      startColumn: location.charRange?.start || 0,
+      endColumn: location.charRange?.end || 1000,
+    },
+  };
+};
+
 /**
  * Convert a DocumentRange to use 1-indexed values. Used since language engines
  * use 0-indexed ranges but Monaco requires 1-indexed ranges.