diff --git a/docs/grammar.txt b/docs/grammar.txt
index 8090378..32d8fcd 100644
--- a/docs/grammar.txt
+++ b/docs/grammar.txt
@@ -13,6 +13,9 @@ Non-terminals are lowercase words.
 
 program         -> declaration* EOF ;
 
+/* returns a SlashType, not a an AST node */
+type_hint       -> ":" TYPE ;
+
 newline         -> ( "\n" | ";" )+ ;
 
 expr_promotion  -> newline | "}" | "&&" | "||" ;
@@ -20,11 +23,12 @@ expr_promotion  -> newline | "}" | "&&" | "||" ;
 declaration     -> var_declaration
                 |  and_or ;
 
-var_declaration -> "var" IDENTIFIER "=" expression newline ;
+var_declaration -> "var" IDENTIFIER type_hint? "=" expression newline ;
 
 and_or          -> statement ( ( "&&" | "||" ) statement )* ;
 
 
+
 /* statements */
 
 statement       -> loop_stmt
diff --git a/include/interpreter/ast.h b/include/interpreter/ast.h
index 8f2ac46..7653400 100644
--- a/include/interpreter/ast.h
+++ b/include/interpreter/ast.h
@@ -173,6 +173,7 @@ typedef struct {
 typedef struct {
     StmtType type;
     StrView name;
+    SlashType type_hint; // optional: defaults to SLASH_ANY
     Expr *initializer;
 } VarStmt;
 
diff --git a/include/interpreter/types/slash_value.h b/include/interpreter/types/slash_value.h
index 776aaad..f1b7917 100644
--- a/include/interpreter/types/slash_value.h
+++ b/include/interpreter/types/slash_value.h
@@ -40,7 +40,8 @@ typedef enum {
     SLASH_TUPLE,
     SLASH_MAP,
     SLASH_NONE,
-    SLASH_TYPE_COUNT,
+    SLASH_TYPE_COUNT, /* not any actual type */
+    SLASH_ANY, /* special type that a value cannot hold in runtime */
 } SlashType;
 
 #define SLASH_TYPE_DYNAMIC(slash_type) \
diff --git a/src/interpreter/interpreter.c b/src/interpreter/interpreter.c
index 4907ef3..867a910 100644
--- a/src/interpreter/interpreter.c
+++ b/src/interpreter/interpreter.c
@@ -430,6 +430,10 @@ static void exec_expr(Interpreter *interpreter, ExpressionStmt *stmt)
 static void exec_var(Interpreter *interpreter, VarStmt *stmt)
 {
     SlashValue value = eval(interpreter, stmt->initializer);
+    if (stmt->type_hint != SLASH_ANY && stmt->type_hint != value.type) {
+	report_runtime_error("Type mismatch. Expected X, but got Y.");
+	ASSERT_NOT_REACHED;
+    }
     var_define(interpreter->scope, &stmt->name, &value);
 }
 
diff --git a/src/interpreter/parser.c b/src/interpreter/parser.c
index 9432b2b..c29d1e1 100644
--- a/src/interpreter/parser.c
+++ b/src/interpreter/parser.c
@@ -33,6 +33,7 @@ static void report_err_and_sync(Parser *parser, char *err_msg);
  * non-terminal grammar rule functions
  */
 
+static SlashType type_hint(Parser *parser);
 static void newline(Parser *parser);
 static void expr_promotion(Parser *parser);
 static Stmt *declaration(Parser *parser);
@@ -216,6 +217,16 @@ static void report_err_and_sync(Parser *parser, char *err_msg)
 
 
 /* grammar functions */
+static SlashType type_hint(Parser *parser)
+{
+    /* came from ':' */
+    if (!match(parser, t_num, t_str, t_bool)) {
+	report_err_and_sync(parser, "Expected type keyword after ':'");
+	return SLASH_ANY;
+    }
+    return token_type_to_slash_type(previous(parser)->type);
+}
+
 static void newline(Parser *parser)
 {
     consume(parser, t_newline, "Expected newline or semicolon");
@@ -248,13 +259,20 @@ static Stmt *declaration(Parser *parser)
 static Stmt *var_decl(Parser *parser)
 {
     /* came from 'var' */
+    VarStmt *stmt = (VarStmt *)stmt_alloc(parser->ast_arena, STMT_VAR);
     Token *name = consume(parser, t_ident, "Expected variable name");
+    stmt->name = name->lexeme;
+
+    /* optional type */
+    if (match(parser, t_colon))
+	stmt->type_hint = type_hint(parser);
+    else
+	stmt->type_hint = SLASH_ANY;
+
     consume(parser, t_equal, "Expected variable definition");
     Expr *initializer = expression(parser);
     expr_promotion(parser);
 
-    VarStmt *stmt = (VarStmt *)stmt_alloc(parser->ast_arena, STMT_VAR);
-    stmt->name = name->lexeme;
     stmt->initializer = initializer;
     return (Stmt *)stmt;
 }
diff --git a/src/test.slash b/src/test.slash
index be0a144..b707d88 100644
--- a/src/test.slash
+++ b/src/test.slash
@@ -1,6 +1,2 @@
-var list1 = ["Nicolai", "Callum", 2, "Aidan", 4, "Zach", "Brendan", 1]
-$list1.sort()
-var i = 0
-loop item in $list1 {
-    echo $item
-}
+var s: str = "Nicolai"
+echo $s