diff --git a/src/lib/expr.test.ts b/src/lib/expr.test.ts
index e998605..3ed361f 100644
--- a/src/lib/expr.test.ts
+++ b/src/lib/expr.test.ts
@@ -11,6 +11,11 @@ describe("expr", () => {
       ["1 * 2", "2"],
       ["1 / 2", "0.5"],
       ["1 // 2", "0"],
+      ["1 % 2", "1"],
+      ["3 % 2", "1"],
+      ["-1 % 2", "1"],
+      // TODO: might change this! negative dividend is weird no matter what, but positive modulo is arguably the better call?
+      ["1 % -2", "-1"],
       // TODO: operator precedence
       // TODO: parentheses
     ])("%s", (expression, result) => {
diff --git a/src/lib/expr.ts b/src/lib/expr.ts
index 00dce77..9d6afb2 100644
--- a/src/lib/expr.ts
+++ b/src/lib/expr.ts
@@ -32,6 +32,7 @@ const MINUS_TOKEN = /\s*(\-)/y;
 const TIMES_TOKEN = /\s*(\*)/y;
 const FLOOR_TOKEN = /\s*(\/\/)/y;
 const DIV_TOKEN = /\s*(\/)/y;
+const MOD_TOKEN = /\s*(\%)/y;
 
 const NUMBER_TOKEN = /\s*(\d+)/y;
 
@@ -84,6 +85,7 @@ const Operators: TokenHandler[] = [
   makeInfixOp(TIMES_TOKEN, 20, 21, (left, right) => ({ value: left * right })),
   makeInfixOp(FLOOR_TOKEN, 20, 21, (left, right) => ({ value: Math.floor(left / right) })),
   makeInfixOp(DIV_TOKEN, 20, 21, (left, right) => ({ value: left / right })),
+  makeInfixOp(MOD_TOKEN, 20, 21, (left, right) => ({ value: ((left % right) + right) % right })),
 ];
 
 const ZERO = { value: 0 };