diff --git a/src/errors.cc b/src/errors.cc
index 0f90965..b0f7fbb 100644
--- a/src/errors.cc
+++ b/src/errors.cc
@@ -71,6 +71,7 @@ std::ostream& operator<<(std::ostream& os, Error const& err)
 
 	case errors::Function_Not_Defined:
 	case errors::Unexpected_Token_Type:
+	case errors::Unresolved_Operator:
 		return os << err.message << '\n';
 
 	case errors::Unexpected_Empty_Source:
@@ -153,6 +154,15 @@ Error errors::function_not_defined(Value const& value)
 	return err;
 }
 
+Error errors::unresolved_operator(Token const& op)
+{
+	Error err;
+	err.type = errors::Unresolved_Operator;
+	err.location = op.location;
+	err.message = format("Unresolved operator '", op.source, "'");
+	return err;
+}
+
 void errors::all_tokens_were_not_parsed(std::span<Token> tokens)
 {
 	error_heading(std::cerr, std::nullopt, Error_Level::Bug);
diff --git a/src/interpreter.cc b/src/interpreter.cc
index e61317a..93e6252 100644
--- a/src/interpreter.cc
+++ b/src/interpreter.cc
@@ -2,6 +2,19 @@
 
 #include <iostream>
 
+static auto numeric_binary_operator(auto binop)
+{
+	return [binop = std::move(binop)](std::vector<Value> args) {
+		auto result = std::move(args.front());
+		for (auto &v : std::span(args).subspan(1)) {
+			assert(result.type == Value::Type::Number, "LHS should be a number");
+			assert(v.type == Value::Type::Number,      "RHS should be a number");
+			result.n = binop(std::move(result.n), v.n);
+		}
+		return result;
+	};
+}
+
 Interpreter::Interpreter()
 	: Interpreter(std::cout)
 {
@@ -20,7 +33,7 @@ Interpreter::Interpreter(std::ostream& out)
 		unreachable();
 	};
 
-	functions["say"] = [&](std::vector<Value> args) -> Value {
+	functions["say"] = [&out](std::vector<Value> args) -> Value {
 		for (auto it = args.begin(); it != args.end(); ++it) {
 			out << *it;
 			if (std::next(it) != args.end())
@@ -29,6 +42,11 @@ Interpreter::Interpreter(std::ostream& out)
 		out << '\n';
 		return {};
 	};
+
+	operators["+"] = numeric_binary_operator(std::plus<>{});
+	operators["-"] = numeric_binary_operator(std::minus<>{});
+	operators["*"] = numeric_binary_operator(std::multiplies<>{});
+	operators["/"] = numeric_binary_operator(std::divides<>{});
 }
 
 Result<Value> Interpreter::eval(Ast &&ast)
@@ -41,28 +59,18 @@ Result<Value> Interpreter::eval(Ast &&ast)
 		{
 			std::vector<Value> values;
 			values.reserve(ast.arguments.size());
+
+			auto op = operators.find(std::string(ast.token.source));
+
+			if (op == operators.end()) {
+				return errors::unresolved_operator(ast.token);
+			}
+
 			for (auto& a : ast.arguments) {
 				values.push_back(Try(eval(std::move(a))));
 			}
 
-			Value result = values.front();
-			if (ast.token.source == "+") {
-				for (auto &v : std::span(values).subspan(1)) {
-					assert(result.type == Value::Type::Number, "LHS of + should be a number");
-					assert(v.type == Value::Type::Number,      "RHS of + should be a number");
-					result.n += v.n;
-				}
-				return result;
-			} else if (ast.token.source == "*") {
-				for (auto &v : std::span(values).subspan(1)) {
-					assert(result.type == Value::Type::Number, "LHS of * should be a number");
-					assert(v.type == Value::Type::Number,      "RHS of * should be a number");
-					result.n *= v.n;
-				}
-				return result;
-			}
-
-			unimplemented();
+			return op->second(std::move(values));
 		}
 		break;
 
diff --git a/src/musique.hh b/src/musique.hh
index d457323..70f2cf0 100644
--- a/src/musique.hh
+++ b/src/musique.hh
@@ -45,7 +45,8 @@ namespace errors
 		Unexpected_Empty_Source,
 		Failed_Numeric_Parsing,
 
-		Function_Not_Defined
+		Function_Not_Defined,
+		Unresolved_Operator
 	};
 }
 
@@ -416,6 +417,7 @@ struct Interpreter
 {
 	std::ostream &out;
 	std::unordered_map<std::string, Function> functions;
+	std::unordered_map<std::string, Function> operators;
 
 	Interpreter();
 	Interpreter(std::ostream& out);
@@ -437,6 +439,7 @@ namespace errors
 	Error failed_numeric_parsing(Location location, std::errc errc, std::string_view source);
 
 	Error function_not_defined(Value const& v);
+	Error unresolved_operator(Token const& op);
 
 	[[noreturn]]
 	void all_tokens_were_not_parsed(std::span<Token>);
diff --git a/src/tests/interpreter.cc b/src/tests/interpreter.cc
index 9ac3c49..f4bbe25 100644
--- a/src/tests/interpreter.cc
+++ b/src/tests/interpreter.cc
@@ -46,6 +46,8 @@ suite intepreter_test = [] {
 		should("evaluate arithmetic") = [] {
 			evaluates_to(Value::number(Number(10)), "5 + 3 + 2");
 			evaluates_to(Value::number(Number(25)), "5 * (3 + 2)");
+			evaluates_to(Value::number(Number(1, 2)), "1 / 2");
+			evaluates_to(Value::number(Number(-10)), "10 - 20");
 		};
 
 		should("call builtin functions") = [] {