diff --git a/example/hello.wren b/example/hello.wren
index 26c74f71..2a70066a 100644
--- a/example/hello.wren
+++ b/example/hello.wren
@@ -1,9 +1,6 @@
-class Foo { // line comment
-  bar {
-    123
-  }
-}
-var a = Foo.new
-a.bar
-"something".contains("meth")
-io.write("hey there")
+io.write(1 + 2)
+io.write(3 - 1)
+io.write(10 / 3)
+io.write(20 * 30)
+io.write("abc" + "def")
+io.write(1 + "string")
diff --git a/src/compiler.c b/src/compiler.c
index 5ad11448..47f1adee 100644
--- a/src/compiler.c
+++ b/src/compiler.c
@@ -135,6 +135,8 @@ static void error(Compiler* compiler, const char* format, ...);
 
 static void statement(Compiler* compiler);
 static void expression(Compiler* compiler);
+static void term(Compiler* compiler);
+static void factor(Compiler* compiler);
 static void call(Compiler* compiler);
 static void primary(Compiler* compiler);
 static void number(Compiler* compiler);
@@ -387,8 +389,61 @@ void statement(Compiler* compiler)
 }
 
 void expression(Compiler* compiler)
+{
+  term(compiler);
+}
+
+void term(Compiler* compiler)
+{
+  factor(compiler);
+
+  while (match(compiler, TOKEN_PLUS) || match(compiler, TOKEN_MINUS))
+  {
+    const char* name;
+    if (compiler->parser->previous.type == TOKEN_PLUS)
+    {
+      name = "+ ";
+    }
+    else
+    {
+      name = "- ";
+    }
+
+    // Compile the right-hand side.
+    factor(compiler);
+
+    // Call the operator method on the left-hand side.
+    int symbol = ensureSymbol(&compiler->parser->vm->symbols, name, 2);
+    emit(compiler, CODE_CALL_1);
+    emit(compiler, symbol);
+  }
+}
+
+// TODO(bob): Virtually identical to term(). Unify.
+void factor(Compiler* compiler)
 {
   call(compiler);
+
+  while (match(compiler, TOKEN_STAR) || match(compiler, TOKEN_SLASH))
+  {
+    const char* name;
+    if (compiler->parser->previous.type == TOKEN_STAR)
+    {
+      name = "* ";
+    }
+    else
+    {
+      name = "/ ";
+    }
+
+    // Compile the right-hand side.
+    call(compiler);
+
+    // Call the operator method on the left-hand side.
+    int symbol = ensureSymbol(&compiler->parser->vm->symbols, name, 2);
+    emit(compiler, CODE_CALL_1);
+    emit(compiler, symbol);
+  }
 }
 
 // Method calls like:
diff --git a/src/primitives.c b/src/primitives.c
index 246fc0d2..ca1b0ba4 100644
--- a/src/primitives.c
+++ b/src/primitives.c
@@ -12,7 +12,7 @@
     }
 
 #define DEF_PRIMITIVE(prim) \
-    static Value primitive_##prim(Value* args, int numArgs)
+    static Value primitive_##prim(VM* vm, Value* args, int numArgs)
 
 #define GLOBAL(cls, name) \
     { \
@@ -29,6 +29,31 @@ DEF_PRIMITIVE(num_abs)
   return (Value)makeNum(value);
 }
 
+DEF_PRIMITIVE(num_minus)
+{
+  if (args[1]->type != OBJ_NUM) return vm->unsupported;
+  return (Value)makeNum(((ObjNum*)args[0])->value - ((ObjNum*)args[1])->value);
+}
+
+DEF_PRIMITIVE(num_plus)
+{
+  if (args[1]->type != OBJ_NUM) return vm->unsupported;
+  // TODO(bob): Handle coercion to string if RHS is a string.
+  return (Value)makeNum(((ObjNum*)args[0])->value + ((ObjNum*)args[1])->value);
+}
+
+DEF_PRIMITIVE(num_multiply)
+{
+  if (args[1]->type != OBJ_NUM) return vm->unsupported;
+  return (Value)makeNum(((ObjNum*)args[0])->value * ((ObjNum*)args[1])->value);
+}
+
+DEF_PRIMITIVE(num_divide)
+{
+  if (args[1]->type != OBJ_NUM) return vm->unsupported;
+  return (Value)makeNum(((ObjNum*)args[0])->value / ((ObjNum*)args[1])->value);
+}
+
 DEF_PRIMITIVE(string_contains)
 {
   const char* string = ((ObjString*)args[0])->value;
@@ -49,6 +74,24 @@ DEF_PRIMITIVE(string_count)
   return (Value)makeNum(count);
 }
 
+DEF_PRIMITIVE(string_plus)
+{
+  if (args[1]->type != OBJ_STRING) return vm->unsupported;
+  // TODO(bob): Handle coercion to string of RHS.
+
+  ObjString* left = (ObjString*)args[0];
+  ObjString* right = (ObjString*)args[1];
+
+  size_t leftLength = strlen(left->value);
+  size_t rightLength = strlen(right->value);
+
+  char* result = malloc(leftLength + rightLength);
+  strcpy(result, left->value);
+  strcpy(result + leftLength, right->value);
+  
+  return (Value)makeString(result);
+}
+
 DEF_PRIMITIVE(io_write)
 {
   printValue(args[1]);
@@ -59,10 +102,21 @@ DEF_PRIMITIVE(io_write)
 void registerPrimitives(VM* vm)
 {
   PRIMITIVE(vm->numClass, "abs", num_abs);
+  PRIMITIVE(vm->numClass, "- ", num_minus);
+  PRIMITIVE(vm->numClass, "+ ", num_plus);
+  PRIMITIVE(vm->numClass, "* ", num_multiply);
+  PRIMITIVE(vm->numClass, "/ ", num_divide);
+
   PRIMITIVE(vm->stringClass, "contains ", string_contains);
   PRIMITIVE(vm->stringClass, "count", string_count);
+  PRIMITIVE(vm->stringClass, "+ ", string_plus);
 
   ObjClass* ioClass = makeClass();
   PRIMITIVE(ioClass, "write ", io_write);
   GLOBAL(ioClass, "io");
+
+  ObjClass* unsupportedClass = makeClass();
+
+  // TODO(bob): Make this a distinct object type.
+  vm->unsupported = (Value)makeInstance(unsupportedClass);
 }
\ No newline at end of file
diff --git a/src/vm.c b/src/vm.c
index a9d6b3e3..a6137454 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -27,9 +27,13 @@ typedef struct
   int numFrames;
 } Fiber;
 
-static Value primitive_metaclass_new(Value* args, int numArgs);
 static void callBlock(Fiber* fiber, ObjBlock* block, int firstLocal);
+static Value primitive_metaclass_new(VM* vm, Value* args, int numArgs);
+
+// Pushes [value] onto the top of the stack.
 static void push(Fiber* fiber, Value value);
+
+// Removes and returns the top of the stack.
 static Value pop(Fiber* fiber);
 
 VM* newVM()
@@ -279,11 +283,10 @@ Value interpret(VM* vm, ObjBlock* block)
       case CODE_CALL_10:
       {
         int numArgs = frame->block->bytecode[frame->ip - 1] - CODE_CALL_0;
-
-        Value receiver = fiber.stack[fiber.stackSize - numArgs - 1];
         int symbol = frame->block->bytecode[frame->ip++];
 
-        // TODO(bob): Support classes for other object types.
+        Value receiver = fiber.stack[fiber.stackSize - numArgs - 1];
+
         ObjClass* classObj;
         switch (receiver->type)
         {
@@ -312,8 +315,11 @@ Value interpret(VM* vm, ObjBlock* block)
         switch (method->type)
         {
           case METHOD_NONE:
-            // TODO(bob): Should return nil or suspend fiber or something.
-            printf("No method.\n");
+            printf("Receiver ");
+            printValue(receiver);
+            printf(" does not implement method \"%s\".\n",
+                   vm->symbols.names[symbol]);
+            // TODO(bob): Throw an exception or halt the fiber or something.
             exit(1);
             break;
 
@@ -322,15 +328,18 @@ Value interpret(VM* vm, ObjBlock* block)
             break;
 
           case METHOD_PRIMITIVE:
-            // TODO(bob): Pass args to primitive.
-            fiber.stack[fiber.stackSize - numArgs - 1] =
-                method->primitive(&fiber.stack[fiber.stackSize - numArgs - 1],
-                                  numArgs);
-
+          {
+            Value* args = &fiber.stack[fiber.stackSize - numArgs - 1];
+            // TODO(bob): numArgs passed to primitive should probably include
+            // receiver.
+            Value result = method->primitive(vm, args, numArgs);
+            fiber.stack[fiber.stackSize - numArgs - 1] = result;
+            
             // Discard the stack slots for the arguments.
-            fiber.stackSize = fiber.stackSize - numArgs;
+            fiber.stackSize -= numArgs;
             break;
-
+          }
+            
           case METHOD_BLOCK:
             callBlock(&fiber, method->block, fiber.stackSize - numArgs);
             break;
@@ -362,14 +371,6 @@ void printValue(Value value)
   // TODO(bob): Do more useful stuff here.
   switch (value->type)
   {
-    case OBJ_NUM:
-      printf("%g", ((ObjNum*)value)->value);
-      break;
-
-    case OBJ_STRING:
-      printf("%s", ((ObjString*)value)->value);
-      break;
-
     case OBJ_BLOCK:
       printf("[block]");
       break;
@@ -381,14 +382,15 @@ void printValue(Value value)
     case OBJ_INSTANCE:
       printf("[instance]");
       break;
-  }
-}
 
-Value primitive_metaclass_new(Value* args, int numArgs)
-{
-  ObjClass* classObj = (ObjClass*)args[0];
-  // TODO(bob): Invoke initializer method.
-  return (Value)makeInstance(classObj);
+    case OBJ_NUM:
+      printf("%g", ((ObjNum*)value)->value);
+      break;
+
+    case OBJ_STRING:
+      printf("%s", ((ObjString*)value)->value);
+      break;
+  }
 }
 
 void callBlock(Fiber* fiber, ObjBlock* block, int firstLocal)
@@ -408,6 +410,13 @@ void callBlock(Fiber* fiber, ObjBlock* block, int firstLocal)
   fiber->numFrames++;
 }
 
+Value primitive_metaclass_new(VM* vm, Value* args, int numArgs)
+{
+  ObjClass* classObj = (ObjClass*)args[0];
+  // TODO(bob): Invoke initializer method.
+  return (Value)makeInstance(classObj);
+}
+
 void push(Fiber* fiber, Value value)
 {
   // TODO(bob): Check for stack overflow.
diff --git a/src/vm.h b/src/vm.h
index 9c30c43c..727b49ec 100644
--- a/src/vm.h
+++ b/src/vm.h
@@ -28,7 +28,9 @@ typedef struct
 
 typedef Obj* Value;
 
-typedef Value (*Primitive)(Value* args, int numArgs);
+typedef struct sVM VM;
+
+typedef Value (*Primitive)(VM* vm, Value* args, int numArgs);
 
 typedef struct
 {
@@ -127,7 +129,7 @@ typedef enum
   CODE_CALL_8,
   CODE_CALL_9,
   CODE_CALL_10,
-
+  
   // The current block is done and should be exited.
   CODE_END
 } Code;
@@ -139,7 +141,7 @@ typedef struct
   int count;
 } SymbolTable;
 
-typedef struct
+struct sVM
 {
   SymbolTable symbols;
 
@@ -148,10 +150,13 @@ typedef struct
   ObjClass* numClass;
   ObjClass* stringClass;
 
+  // The singleton value "unsupported".
+  Value unsupported;
+
   SymbolTable globalSymbols;
   // TODO(bob): Using a fixed array is gross here.
   Value globals[MAX_SYMBOLS];
-} VM;
+};
 
 VM* newVM();
 void freeVM(VM* vm);
diff --git a/test/number_divide.wren b/test/number_divide.wren
new file mode 100644
index 00000000..c27a65aa
--- /dev/null
+++ b/test/number_divide.wren
@@ -0,0 +1,4 @@
+io.write(8 / 2) // expect: 4
+
+// TODO(bob): Floating point numbers.
+// TODO(bob): Unsupported RHS types.
diff --git a/test/number_minus.wren b/test/number_minus.wren
new file mode 100644
index 00000000..86215f07
--- /dev/null
+++ b/test/number_minus.wren
@@ -0,0 +1,5 @@
+io.write(5 - 3) // expect: 2
+io.write(3 - 2 - 1) // expect: 0
+
+// TODO(bob): Floating point numbers.
+// TODO(bob): Unsupported RHS types.
diff --git a/test/number_multiply.wren b/test/number_multiply.wren
new file mode 100644
index 00000000..d5a91e71
--- /dev/null
+++ b/test/number_multiply.wren
@@ -0,0 +1,4 @@
+io.write(5 * 3) // expect: 15
+
+// TODO(bob): Floating point numbers.
+// TODO(bob): Unsupported RHS types.
diff --git a/test/number_plus.wren b/test/number_plus.wren
new file mode 100644
index 00000000..ac413c65
--- /dev/null
+++ b/test/number_plus.wren
@@ -0,0 +1,4 @@
+io.write(3 + 5 + 2) // expect: 10
+
+// TODO(bob): Floating point numbers.
+// TODO(bob): Unsupported RHS types.