diff --git a/README.md b/README.md
index 09d3be7..63888c8 100644
--- a/README.md
+++ b/README.md
@@ -24,8 +24,8 @@ TODO
 * Optimization pass to turn multiply loops into commands that do `x += y * c`
 * Make VM use a Proxied object that gives out 0 for nonexistent elements for
   its memory
-* Implement UTF-8 I/O
 * Keep a cache of compiled programs in `run()`
+* Support for other types of EOF?
 
 ### gir.html
 * Implement a UI
diff --git a/gir.js b/gir.js
index 2296f16..8ae5fdc 100644
--- a/gir.js
+++ b/gir.js
@@ -442,7 +442,7 @@ function optimize(parsed) {
 // Virtual machine
 // ------------------------------------------------------------------
 
-// ([flatCommandObject]) → girVMState
+// ([flatCommandObject], [int]) → girVMState
 function newVM(program, input) {
 	return {
 		// Initial state for the machine
@@ -453,7 +453,7 @@ function newVM(program, input) {
 		tapeHead: 0,
 
 		input: input,
-		output: ''
+		output: []
 	};
 }
 
@@ -474,8 +474,9 @@ function runVM(state, maxCycles = null) {
 	}
 	let tapeHead = state.tapeHead;
 
-	let input = state.input;
-	let output = state.output;
+	// Create copies of input and output, since we might modify them
+	let input = state.input.slice();
+	let output = state.output.slice();
 
 	let complete = false;
 	let cycle = 0;
@@ -526,22 +527,18 @@ function runVM(state, maxCycles = null) {
 
 			case writeByte:
 				if(!(index in memory)) memory[index] = 0;
-				// TODO: utf-8
-				output += String.fromCodePoint(memory[index]);
+				output.push(memory[index]);
 				ip++;
 				break;
 
 			case readByte:
-				// TODO: utf-8
 				// Have we reached EOF?
 				if(input.length == 0) {
 					// Yes, return 0
 					memory[index] = 0;
 				} else {
 					// No, return character
-					memory[index] = input.codePointAt(0);
-					// FIXME: This only works for BMP
-					input = input.slice(1);
+					memory[index] = input.shift();
 				}
 				ip++;
 				break;
@@ -589,6 +586,117 @@ function runVM(state, maxCycles = null) {
 	return {state: newState, complete: complete, cycles: cycle};
 }
 
+// ------------------------------------------------------------------
+// UTF-8
+// ------------------------------------------------------------------
+
+// string → [int]
+function encodeUTF8(string) {
+	let encoded = [];
+
+	for(let character of string) {
+		let codepoint = character.codePointAt(0);
+
+		if(codepoint < 0x80) {
+			// 0xxxxxxx
+			encoded.push(codepoint);
+		} else if(codepoint < 0x0800) {
+			// 110xxxxx 10xxxxxx
+			let b1 = codepoint >> 6 | 0b11000000;
+			let b2 = codepoint & 0b00111111 | 0b10000000;
+			encoded.push(b1);
+			encoded.push(b2);
+		} else if(codepoint < 0x10000) {
+			// 1110xxxx 10xxxxxx 10xxxxxx
+			let b1 = codepoint >> 12 | 0b11100000;
+			let b2 = codepoint >> 6 & 0b00111111 | 0b10000000;
+			let b3 = codepoint & 0b00111111 | 0b10000000;
+			encoded.push(b1);
+			encoded.push(b2);
+			encoded.push(b3);
+		} else {
+			// 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
+			let b1 = codepoint >> 18 | 0b11110000;
+			let b2 = codepoint >> 12 & 0b00111111 | 0b10000000;
+			let b3 = codepoint >> 6 & 0b00111111 | 0b10000000;
+			let b4 = codepoint & 0b00111111 | 0b10000000;
+			encoded.push(b1);
+			encoded.push(b2);
+			encoded.push(b3);
+			encoded.push(b4);
+		}
+	}
+
+	return encoded;
+}
+
+// [int] → string
+function decodeUTF8(encoded) {
+	let codePoints = [];
+
+	for(let i = 0; i < encoded.length;) {
+		let codePoint = 0;
+
+		let firstByte = encoded[i];
+		i++;
+
+		let toRead = null;
+		// Determine number of continuation bytes to read and
+		// decode the first byte into codePoint
+		// Since we'll do the shifts later, we just mask here
+		if(firstByte >> 7 == 0) {
+			// 0xxxxxxx
+			toRead = 0;
+			codePoint = firstByte;
+		} else if(firstByte >> 5 == 0b110) {
+			// 110xxxxx 10xxxxxx
+			toRead = 1;
+			codePoint = firstByte & 0b00011111;
+		} else if(firstByte >> 4 == 0b1110) {
+			// 1110xxxx 10xxxxxx 10xxxxxx
+			toRead = 2;
+			codePoint = firstByte & 0b00001111;
+		} else if(firstByte >> 3 == 0b11110) {
+			// 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
+			toRead = 3;
+			codePoint = firstByte & 0b00000111;
+		} else {
+			// Illegal sequence, push replacement char
+			codePoints.push(0xFFFD);
+			continue;
+		}
+
+		for(; toRead > 0 && i < encoded.length; toRead--) {
+			let continuationByte = encoded[i];
+			i++;
+
+			// Check that we have a valid continuation byte
+			if(continuationByte >> 6 == 0b10) {
+				// We do, add its contents to codePoint
+				codePoint = codePoint << 6 |
+					continuationByte & 0b00111111;
+			} else {
+				// We don't, break out of the loop
+				break;
+			}
+		}
+
+		// Did we read all required continuation bytes?
+		if(toRead == 0) {
+			// We did, add the codepoint to the array
+			codePoints.push(codePoint);
+		} else {
+			// We didn't, push replacement char
+			codePoints.push(0xFFFD);
+		}
+	}
+
+	// Convert to a string
+	let decoded = codePoints.map(x => String.fromCodePoint(x)).join('');
+
+	return decoded;
+}
+
 // ------------------------------------------------------------------
 // User-facing functions
 // ------------------------------------------------------------------
@@ -598,14 +706,14 @@ function compile(program) {
 	return optimize(parse(program));
 }
 
-// (string, string, bool) → string
+// (string, string, int) → string
 function run(program, input, maxCycles = null) {
 	// TODO; Cache programs
 	let compiled = compile(program);
-	let vm = newVM(compiled, input);
+	let vm = newVM(compiled, encodeUTF8(input));
 
 	let result = runVM(vm, maxCycles);
-	let output = result.state.output;
+	let output = decodeUTF8(result.state.output);
 
 	// If didn't complete, mark it in the output
 	if(!result.complete) {