diff --git a/src/compiler.c b/src/compiler.c
index 6088a70..346e949 100644
--- a/src/compiler.c
+++ b/src/compiler.c
@@ -2724,43 +2724,46 @@ static void string(int exprType) {
 				krk_rewindScanner(beforeExpression); /* To get us back to where we were with a string token */
 				parser = parserBefore;
 				c = inner.start;
-				KrkToken which = syntheticToken("str");
-				int hasEq = 0;
+
+				int formatType = 0;
+
 				while (*c == ' ') c++;
 				if (*c == '=') {
 					c++;
 					while (*c == ' ') c++;
 					emitConstant(OBJECT_VAL(krk_copyString(start,c-start)));
-					emitByte(OP_SWAP);
-					hasEq = 1;
+					formatType |= FORMAT_OP_EQ;
 				}
+
 				if (*c == '!') {
 					c++;
 					/* Conversion specifiers, must only be one */
 					if (*c == 'r') {
-						which = syntheticToken("repr");
+						formatType |= FORMAT_OP_REPR;
 					} else if (*c == 's') {
-						which = syntheticToken("str");
+						formatType |= FORMAT_OP_STR;
 					} else {
 						error("Unsupported conversion flag '%c' for f-string expression.", *c);
 						goto _cleanupError;
 					}
 					c++;
 				}
-				size_t ind = identifierConstant(&which);
-				EMIT_OPERAND_OP(OP_GET_GLOBAL, ind);
-				emitByte(OP_SWAP);
-				emitBytes(OP_CALL, 1);
+
 				if (*c == ':') {
 					/* TODO format specs */
-					error("Format spec not supported in f-string (GH-10)");
-					goto _cleanupError;
+					const char * formatStart = c+1;
+					c++;
+					while (c < end && *c != '}') c++;
+					emitConstant(OBJECT_VAL(krk_copyString(formatStart,c-formatStart)));
+					formatType |= FORMAT_OP_FORMAT;
 				}
+
+				EMIT_OPERAND_OP(OP_FORMAT_VALUE, formatType);
+
 				if (*c != '}') {
 					error("Expected closing '}' after expression in f-string");
 					goto _cleanupError;
 				}
-				if (hasEq) emitByte(OP_ADD);
 				if (atLeastOne) emitByte(OP_ADD);
 				atLeastOne = 1;
 				c++;
diff --git a/src/kuroko/chunk.h b/src/kuroko/chunk.h
index c88dce6..95ae8a1 100644
--- a/src/kuroko/chunk.h
+++ b/src/kuroko/chunk.h
@@ -126,6 +126,7 @@ typedef enum {
 	OP_CALL_METHOD,
 	OP_CLOSE_MANY,
 	OP_POP_MANY,
+	OP_FORMAT_VALUE,
 
 	/* Two opcode instructions */
 	OP_JUMP_IF_FALSE_OR_POP,
@@ -177,6 +178,7 @@ typedef enum {
 	OP_CALL_METHOD_LONG,
 	OP_CLOSE_MANY_LONG,
 	OP_POP_MANY_LONG,
+	OP_FORMAT_VALUE_LONG, /* should be unused */
 } KrkOpCode;
 
 /**
diff --git a/src/opcodes.h b/src/opcodes.h
index 8f0a32a..416d055 100644
--- a/src/opcodes.h
+++ b/src/opcodes.h
@@ -97,6 +97,7 @@ OPERAND(OP_SLICE, (void)0)
 OPERAND(OP_CALL_METHOD, (void)0)
 OPERAND(OP_CLOSE_MANY, (void)0)
 OPERAND(OP_POP_MANY, (void)0)
+OPERAND(OP_FORMAT_VALUE, (void)0)
 JUMP(OP_JUMP_IF_FALSE_OR_POP,+)
 JUMP(OP_JUMP_IF_TRUE_OR_POP,+)
 JUMP(OP_JUMP,+)
diff --git a/src/private.h b/src/private.h
index d078b8d..c7f3bce 100644
--- a/src/private.h
+++ b/src/private.h
@@ -51,3 +51,10 @@ typedef enum {
 	METHOD__MAX,
 } KrkSpecialMethods;
 
+
+#define FORMAT_OP_EQ     (1 << 0)
+#define FORMAT_OP_REPR   (1 << 1)
+#define FORMAT_OP_STR    (1 << 2)
+#define FORMAT_OP_FORMAT (1 << 3)
+
+
diff --git a/src/vm.c b/src/vm.c
index b9db161..c255a5e 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -2537,6 +2537,74 @@ static inline void makeCollection(NativeFn func, size_t count) {
 	}
 }
 
+static inline int doFormatString(int options) {
+	if (options & FORMAT_OP_FORMAT) {
+		krk_swap(1);
+		if (options & FORMAT_OP_EQ) {
+			krk_swap(2);
+		}
+	} else if (options & FORMAT_OP_EQ) {
+		krk_swap(1);
+	}
+
+	/* Was this a repr or str call? (it can't be both) */
+	if (options & FORMAT_OP_STR) {
+		KrkClass * type = krk_getType(krk_peek(0));
+		if (likely(type->_tostr != NULL)) {
+			krk_push(krk_callDirect(type->_tostr, 1));
+			if (unlikely(krk_currentThread.flags & KRK_THREAD_HAS_EXCEPTION)) return 1;
+		} else {
+			krk_runtimeError(vm.exceptions->typeError,
+				"Can not convert %s to str", krk_typeName(krk_peek(0)));
+			return 1;
+		}
+	} else if (options & FORMAT_OP_REPR) {
+		KrkClass * type = krk_getType(krk_peek(0));
+		if (likely(type->_reprer != NULL)) {
+			krk_push(krk_callDirect(type->_reprer, 1));
+			if (unlikely(krk_currentThread.flags & KRK_THREAD_HAS_EXCEPTION)) return 1;
+		} else {
+			krk_runtimeError(vm.exceptions->typeError,
+				"Can not repr %s", krk_typeName(krk_peek(0)));
+			return 1;
+		}
+	}
+
+	if (!(options & FORMAT_OP_FORMAT)) {
+		/* Push empty string */
+		krk_push(OBJECT_VAL(S("")));
+	} else {
+		/* Swap args so value is first */
+		krk_swap(1);
+	}
+
+	/* Get the type of the value */
+	KrkClass * type = krk_getType(krk_peek(1));
+
+	if (likely(type->_format != NULL)) {
+		krk_push(krk_callDirect(type->_format, 2));
+		if (unlikely(krk_currentThread.flags & KRK_THREAD_HAS_EXCEPTION)) return 1;
+	} else {
+		krk_runtimeError(vm.exceptions->attributeError, "'%s' object has no attribute '%s'", krk_typeName(krk_peek(1)), "__format__");
+		return 1;
+	}
+
+	if (!IS_STRING(krk_peek(0))) {
+		krk_runtimeError(vm.exceptions->typeError, "format result is not str");
+		return 1;
+	}
+
+	if (options & FORMAT_OP_EQ) {
+		krk_push(krk_operator_add(krk_peek(1), krk_peek(0)));
+		krk_swap(2);
+		krk_pop();
+		krk_pop();
+	}
+
+	return 0;
+}
+
+
 /**
  * VM main loop.
  */
@@ -3429,6 +3497,14 @@ _finishReturn: (void)0;
 				krk_currentThread.stackTop[-count] = values->values.values[0];
 				break;
 			}
+
+			case OP_FORMAT_VALUE_LONG:
+				THREE_BYTE_OPERAND;
+			case OP_FORMAT_VALUE: {
+				ONE_BYTE_OPERAND;
+				if (doFormatString(OPERAND)) goto _finishException;
+				break;
+			}
 		}
 		if (unlikely(krk_currentThread.flags & KRK_THREAD_HAS_EXCEPTION)) {
 _finishException:
diff --git a/test/testFormatSpecs.krk b/test/testFormatSpecs.krk
new file mode 100644
index 0000000..05ef36d
--- /dev/null
+++ b/test/testFormatSpecs.krk
@@ -0,0 +1,2 @@
+print(f'{4225325:#016_x}')
+print(f'{4225325325325325325632632532432432:#016_x}')
diff --git a/test/testFormatSpecs.krk.expect b/test/testFormatSpecs.krk.expect
new file mode 100644
index 0000000..9c42e92
--- /dev/null
+++ b/test/testFormatSpecs.krk.expect
@@ -0,0 +1,2 @@
+0x0000_0040_792d
+0xd053_1a76_d6a2_ecdd_ed78_bd72_f630