diff --git a/Cargo.lock b/Cargo.lock
index ba05e06..64301cb 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -92,7 +92,7 @@ dependencies = [
 
 [[package]]
 name = "kalk"
-version = "0.2.2"
+version = "0.2.3"
 dependencies = [
  "lazy_static",
  "phf",
@@ -103,7 +103,7 @@ dependencies = [
 
 [[package]]
 name = "kalk_cli"
-version = "0.3.2"
+version = "0.3.3"
 dependencies = [
  "ansi_term",
  "kalk",
diff --git a/kalk/Cargo.toml b/kalk/Cargo.toml
index 1ac72c7..505ece8 100644
--- a/kalk/Cargo.toml
+++ b/kalk/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "kalk"
-version = "0.2.2"
+version = "0.2.3"
 authors = ["PaddiM8"]
 edition = "2018"
 readme = "README.md"
diff --git a/kalk/src/interpreter.rs b/kalk/src/interpreter.rs
index f238e8b..2e2f37c 100644
--- a/kalk/src/interpreter.rs
+++ b/kalk/src/interpreter.rs
@@ -113,7 +113,7 @@ fn eval_binary_expr(
     }
 
     let (left, left_unit) = eval_expr(context, left_expr, "")?;
-    let (right, _) = if left_unit.len() > 0 {
+    let (mut right, _) = if left_unit.len() > 0 {
         let (_, right_unit) = eval_expr(context, right_expr, "")?; // TODO: Avoid evaluating this twice.
 
         if right_unit.len() > 0 {
@@ -131,12 +131,17 @@ fn eval_binary_expr(
         unit.into()
     };
 
+    if let Expr::Unary(TokenKind::Percent, _) = right_expr {
+        right *= left.clone();
+    }
+
     Ok((
         match op {
             TokenKind::Plus => left + right,
             TokenKind::Minus => left - right,
             TokenKind::Star => left * right,
             TokenKind::Slash => left / right,
+            TokenKind::Percent => left % right,
             TokenKind::Power => left.pow(right),
             _ => Float::with_val(1, 1),
         },
@@ -154,6 +159,7 @@ fn eval_unary_expr(
 
     match op {
         TokenKind::Minus => Ok((-expr_value, unit)),
+        TokenKind::Percent => Ok((expr_value * 0.01, unit)),
         TokenKind::Exclamation => Ok((
             Float::with_val(
                 context.precision,
@@ -409,6 +415,17 @@ mod tests {
         assert_eq!(interpret(pow).unwrap().unwrap(), 8);
     }
 
+    #[test]
+    fn test_percent() {
+        let stmt = Stmt::Expr(binary(
+            literal("5"),
+            Percent,
+            group(binary(literal("3"), Star, unary(Percent, literal("2")))),
+        ));
+
+        assert!(cmp(interpret(stmt).unwrap().unwrap(), 0.14));
+    }
+
     #[test]
     fn test_unary() {
         let neg = Stmt::Expr(unary(Minus, literal("1")));
diff --git a/kalk/src/lexer.rs b/kalk/src/lexer.rs
index 590c2b6..35c856a 100644
--- a/kalk/src/lexer.rs
+++ b/kalk/src/lexer.rs
@@ -15,6 +15,7 @@ pub enum TokenKind {
     Power,
     Equals,
     Exclamation,
+    Percent,
 
     UnitKeyword,
     ToKeyword,
@@ -112,6 +113,7 @@ impl<'a> Lexer<'a> {
             '!' => build(TokenKind::Exclamation, "", span),
             ',' => build(TokenKind::Comma, "", span),
             ';' => build(TokenKind::Semicolon, "", span),
+            '%' => build(TokenKind::Percent, "", span),
             _ => build(TokenKind::Unknown, "", span),
         };
 
@@ -207,7 +209,7 @@ fn build(kind: TokenKind, value: &str, span: (usize, usize)) -> Token {
 
 fn is_valid_identifier(c: Option<&char>) -> bool {
     if let Some(c) = c {
-        regex::Regex::new(r"[^\s\n\r0-9\+-/\*\^!\(\)=\.,;|⌊⌋⌈⌉]")
+        regex::Regex::new(r"[^\s\n\r0-9\+-/%\*\^!\(\)=\.,;|⌊⌋⌈⌉]")
             .unwrap()
             .is_match(&c.to_string())
     } else {
@@ -230,12 +232,13 @@ mod tests {
 
     #[test]
     fn test_token_kinds() {
-        let tokens = Lexer::lex("+-*/^()|=!,");
+        let tokens = Lexer::lex("+-*/%^()|=!,");
         let expected = vec![
             TokenKind::Plus,
             TokenKind::Minus,
             TokenKind::Star,
             TokenKind::Slash,
+            TokenKind::Percent,
             TokenKind::Power,
             TokenKind::OpenParenthesis,
             TokenKind::ClosedParenthesis,
diff --git a/kalk/src/parser.rs b/kalk/src/parser.rs
index 6c94e0f..f897ce2 100644
--- a/kalk/src/parser.rs
+++ b/kalk/src/parser.rs
@@ -72,6 +72,7 @@ pub enum CalcError {
     UndefinedFn(String),
     UndefinedVar(String),
     UnableToInvert(String),
+    UnableToParseExpression,
     Unknown,
 }
 
@@ -240,18 +241,41 @@ fn parse_sum(context: &mut Context) -> Result<Expr, CalcError> {
 fn parse_factor(context: &mut Context) -> Result<Expr, CalcError> {
     let mut left = parse_unit(context)?;
 
+    if let Expr::Unary(TokenKind::Percent, percent_left) = left.clone() {
+        let try_parse = parse_factor(context);
+        if !try_parse.is_err() {
+            left = Expr::Binary(
+                percent_left,
+                TokenKind::Percent,
+                Box::new(try_parse.unwrap()),
+            );
+        }
+    }
+
     while match_token(context, TokenKind::Star)
         || match_token(context, TokenKind::Slash)
+        || match_token(context, TokenKind::Percent)
         || match_token(context, TokenKind::Identifier)
         || match_token(context, TokenKind::Literal)
     {
-        // If the next token is an identifier, assume it's multiplication. Eg. 3y
+        // If the token is an identifier, assume it's multiplication. Eg. 3y
         let op = match peek(context).kind {
             TokenKind::Identifier | TokenKind::Literal => TokenKind::Star,
             _ => advance(context).kind.clone(),
         };
 
-        let right = parse_unit(context)?;
+        let parse_next = parse_unit(context);
+        let right = if let Ok(right) = parse_next {
+            right
+        /*} else if let Err(CalcError::UnableToParseExpression) = parse_next {
+        // If it failed to parse further,
+        // try to parse it as something else.
+        // Eg. percent unary
+        break;*/
+        } else {
+            return parse_next;
+        };
+
         left = Expr::Binary(Box::new(left), op, Box::new(right));
     }
 
@@ -279,7 +303,12 @@ fn parse_unary(context: &mut Context) -> Result<Expr, CalcError> {
         return Ok(Expr::Unary(op, expr));
     }
 
-    Ok(parse_exponent(context)?)
+    let expr = parse_exponent(context)?;
+    if match_token(context, TokenKind::Percent) {
+        Ok(Expr::Unary(advance(context).kind.clone(), Box::new(expr)))
+    } else {
+        Ok(expr)
+    }
 }
 
 fn parse_exponent(context: &mut Context) -> Result<Expr, CalcError> {
@@ -310,7 +339,8 @@ fn parse_primary(context: &mut Context) -> Result<Expr, CalcError> {
         TokenKind::OpenParenthesis => parse_group(context)?,
         TokenKind::Pipe | TokenKind::OpenCeil | TokenKind::OpenFloor => parse_group_fn(context)?,
         TokenKind::Identifier => parse_identifier(context)?,
-        _ => Expr::Literal(advance(context).value.clone()),
+        TokenKind::Literal => Expr::Literal(advance(context).value.clone()),
+        _ => return Err(CalcError::UnableToParseExpression),
     };
 
     Ok(expr)
@@ -526,6 +556,28 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_percent() {
+        let tokens = vec![
+            token(Literal, "1"),
+            token(Percent, ""),
+            token(Literal, "1"),
+            token(Plus, ""),
+            token(Literal, "5"),
+            token(Percent, ""),
+            token(EOF, ""),
+        ];
+
+        assert_eq!(
+            parse(tokens).unwrap(),
+            Stmt::Expr(binary(
+                binary(literal("1"), Percent, literal("1"),),
+                Plus,
+                unary(Percent, literal("5"))
+            ))
+        );
+    }
+
     #[test]
     fn test_unit() {
         let tokens = vec![token(Literal, "1"), token(Identifier, "a")];
diff --git a/kalk_cli/Cargo.toml b/kalk_cli/Cargo.toml
index 185914c..db5f2d5 100644
--- a/kalk_cli/Cargo.toml
+++ b/kalk_cli/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "kalk_cli"
-version = "0.3.2"
+version = "0.3.3"
 authors = ["PaddiM8"]
 edition = "2018"
 readme = "../README.md"
@@ -15,7 +15,7 @@ path = "src/main.rs"
 name = "kalk"
 
 [dependencies]
-kalk = { path = "../kalk", version = "^0.2.2" }
+kalk = { path = "../kalk", version = "^0.2.3" }
 rustyline = "6.1.2"
 ansi_term = "0.12"
 regex = "1"
diff --git a/kalk_cli/src/output.rs b/kalk_cli/src/output.rs
index 37649dd..de44d2b 100644
--- a/kalk_cli/src/output.rs
+++ b/kalk_cli/src/output.rs
@@ -61,6 +61,7 @@ fn print_calc_err(err: CalcError) {
         UnableToInvert(msg) => format!("Unable to invert: {}", msg),
         UndefinedFn(name) => format!("Undefined function: '{}'.", name),
         UndefinedVar(name) => format!("Undefined variable: '{}'.", name),
+        UnableToParseExpression => format!("Unable to parse expression."),
         Unknown => format!("Unknown error."),
     });
 }
diff --git a/kalk_cli/src/repl.rs b/kalk_cli/src/repl.rs
index 490836b..e37709a 100644
--- a/kalk_cli/src/repl.rs
+++ b/kalk_cli/src/repl.rs
@@ -55,7 +55,7 @@ impl Highlighter for LineHighlighter {
         let reg = Regex::new(
             r"(?x)
             (?P<identifier>[^!-@\s_|^⌊⌋⌈⌉]+(_\d+)?) |
-            (?P<op>[+\-/*^!])",
+            (?P<op>[+\-/*%^!])",
         )
         .unwrap();