diff --git a/src/Windows12.hs b/src/Windows12.hs
index 185b4a3..8ccebaf 100644
--- a/src/Windows12.hs
+++ b/src/Windows12.hs
@@ -5,3 +5,4 @@ import Windows12.Lexer
 import Windows12.Parser
 import Windows12.CodeGen
 import Windows12.TAst
+import Windows12.Semant
diff --git a/src/Windows12/Semant.hs b/src/Windows12/Semant.hs
new file mode 100644
index 0000000..fb36cd2
--- /dev/null
+++ b/src/Windows12/Semant.hs
@@ -0,0 +1,263 @@
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE OverloadedStrings #-}
+
+module Windows12.Semant where
+
+import Data.Text (Text)
+
+import Control.Monad.State
+
+import Data.List (find)
+
+import Windows12.Ast as Ast
+import Windows12.TAst as TAst
+
+suppliedFuncs :: [Text]
+suppliedFuncs = ["printf"]
+
+-- Convert an Ast to a TAst
+-- Performs type inference and type checking
+
+data Ctx = Ctx { structs :: [TLStruct],
+                 enums :: [TLEnum],
+                 funcs :: [TTLFunc],
+                 vars :: [(Text, Type)] }
+    deriving (Eq, Show)
+
+-- Main conversion function. May return an error message if the program
+-- is not well-typed.
+convert :: Ast.Program -> Either String TAst.TProgram
+convert (Ast.Program structs enums funcs) = do
+    let ctx = Ctx structs enums [] []
+    let (funcs', _) = runState (mapM convertFunc funcs) ctx
+    return $ TAst.TProgram structs enums funcs'
+
+-- Convert a TLFunc (Top Level Function) to a TTLFunc (Typed Top Level Function)
+-- Note that the function must be added to the context before converting statements
+-- of the function. This is because the function may call itself recursively.
+-- After converting the function, the function's statements are converted
+-- and added to the context.
+convertFunc :: MonadState Ctx m => Ast.TLFunc -> m TAst.TTLFunc
+convertFunc (Ast.Func name args retType body) = do
+    args' <- mapM (\(Bind name t) -> return (name, t)) args
+    oldFuncs <- gets funcs
+    modify (\ctx -> ctx { funcs = funcs ctx ++ [TTLFunc name args retType []], vars = args' })
+    body' <- mapM convertStmt body
+    ctx <- get
+    let func = (last $ funcs ctx) { TAst.funcBody = body' }
+    put $ ctx { funcs = oldFuncs ++ [func] }
+    return func
+
+-- Convert a statement
+convertStmt :: MonadState Ctx m => Ast.Stmt -> m TAst.TStmt
+
+convertStmt (Ast.Expr expr) = do
+    expr' <- convertExpr expr
+    return $ TAst.TExprStmt expr'
+
+convertStmt (Ast.Return expr) = do
+    expr' <- convertExpr expr
+    return $ TAst.TReturn expr'
+
+convertStmt (Ast.If cond thenStmts elseStmts) = do
+    thenStmts' <- mapM convertStmt thenStmts
+    elseStmts' <- mapM convertStmt $ maybe [] id elseStmts
+    cond' <- convertExpr cond
+    return $ TAst.TIf cond' thenStmts' (Just elseStmts')
+
+convertStmt (Ast.While cond stmts) = do
+    stmts' <- mapM convertStmt stmts
+    cond' <- convertExpr cond
+    return $ TAst.TWhile cond' stmts'
+
+convertStmt (Ast.Assign op lval expr) = do
+    lval' <- convertLVal lval
+    expr' <- convertExpr expr
+    return $ TAst.TAssign op lval' expr'
+
+convertStmt (Ast.Block stmts) = do
+    stmts' <- mapM convertStmt stmts
+    return $ TAst.TBlock stmts'
+
+convertStmt (Ast.Var name (Just t) maybeExpr) = do
+    expr' <- maybe (return Nothing) (fmap Just . convertExpr) maybeExpr
+    modify (\ctx -> ctx { vars = (name, t) : vars ctx })
+    return $ TAst.TDeclVar name t expr'
+
+-- TODO
+convertStmt (Ast.Var name Nothing maybeExpr) = error "Type inference not implemented"
+
+-- Convert an expression to an LValue
+-- Only certain expressions are allowed as LValues
+convertLVal :: MonadState Ctx m => Ast.Expr -> m TAst.TLVal
+convertLVal (Ast.Id name) = do
+    ctx <- get
+    case lookup name (vars ctx) of
+        Just t -> return (t, TAst.TId name)
+        Nothing -> error $ "Variable " ++ show name ++ " not in scope"
+
+convertLVal (Ast.Index arr idx) = do
+    arr' <- convertLVal arr
+    idx' <- convertExpr idx
+    return (fst arr', TAst.LTIndex arr' idx')
+
+convertLVal (Ast.Member e (Id m)) = do
+    e' <- convertLVal e
+    return (fst e', TAst.LTMember e' m)
+
+convertLVal (Ast.Member e m) = do error $ "Invalid member access " ++ show m ++ " on " ++ show e
+
+convertLVal (Ast.UnOp Ast.Deref e) = error "Dereferencing not implemented"
+
+convertLVal e = do error $ "Invalid or unimplemented LValue " ++ show e
+
+-- Convert an expression
+convertExpr :: MonadState Ctx m => Ast.Expr -> m TAst.TExpr
+convertExpr (Ast.Id name) = do
+    ctx <- get
+    case lookup name (vars ctx) of
+        Just t -> return (t, TAst.TVar name)
+        Nothing -> error $ "Variable " ++ show name ++ " not in scope"
+
+convertExpr (Ast.IntLit x) = return (IntType, TAst.TIntLit x)
+convertExpr (Ast.UIntLit x) = return (UIntType, TAst.TUIntLit x)
+convertExpr (Ast.FloatLit x) = return (FloatType, TAst.TFloatLit x)
+convertExpr (Ast.StrLit x) = return (StrType, TAst.TStrLit x)
+convertExpr (Ast.BoolLit x) = return (BoolType, TAst.TBoolLit x)
+convertExpr (Ast.CharLit x) = return (CharType, TAst.TCharLit x)
+
+convertExpr (Ast.BinOp Add l r) = arithOp Add l r
+convertExpr (Ast.BinOp Sub l r) = arithOp Sub l r
+convertExpr (Ast.BinOp Mul l r) = arithOp Mul l r
+convertExpr (Ast.BinOp Div l r) = arithOp Div l r
+convertExpr (Ast.BinOp Mod l r) = arithOp Mod l r
+
+convertExpr (Ast.BinOp Eq l r) = compOp Eq l r
+convertExpr (Ast.BinOp Ne l r) = compOp Ne l r
+convertExpr (Ast.BinOp Lt l r) = compOp Lt l r
+convertExpr (Ast.BinOp Gt l r) = compOp Gt l r
+convertExpr (Ast.BinOp Le l r) = compOp Le l r
+convertExpr (Ast.BinOp Ge l r) = compOp Ge l r
+
+convertExpr (Ast.BinOp And l r) = boolOp And l r
+convertExpr (Ast.BinOp Or l r) = boolOp Or l r
+
+convertExpr (Ast.BinOp BitAnd l r) = bitOp BitAnd l r
+convertExpr (Ast.BinOp BitOr l r) = bitOp BitOr l r
+convertExpr (Ast.BinOp BitXor l r) = bitOp BitXor l r
+
+convertExpr (Ast.BinOp ShiftL l r) = shiftOp ShiftL l r
+convertExpr (Ast.BinOp ShiftR l r) = shiftOp ShiftR l r
+
+convertExpr (Ast.UnOp Neg e) = do
+    e' <- convertExpr e
+    if fst e' `elem` [IntType, UIntType, FloatType]
+        then return (fst e', TAst.TUnOp Neg e')
+        else error $ "Type mismatch: " ++ show e
+
+convertExpr (Ast.UnOp Not e) = do
+    e' <- convertExpr e
+    if fst e' == BoolType
+        then return (BoolType, TAst.TUnOp Not e')
+        else error $ "Type mismatch: " ++ show e
+
+convertExpr (Ast.UnOp BitNot e) = undefined
+convertExpr (Ast.UnOp Deref e) = undefined
+convertExpr (Ast.UnOp AddrOf e) = undefined
+
+-- TODO type check function return
+-- TODO ensure returns on all paths
+-- Lower priority since LLVM checks this also
+convertExpr (Ast.Call (Id f) args) = do
+    ctx <- get
+    if f == "printf"
+        then do
+            args' <- mapM convertExpr args
+            return (IntType, TAst.TCall "printf" args')
+        else case find (\(TTLFunc n a r _) -> n == f) (funcs ctx) of
+            Just t -> do
+                args' <- mapM convertExpr args
+                if length args' == length (TAst.funcArgs t) && all (\(t1, t2) -> t1 == t2) (zip (map fst args') (map bindType (TAst.funcArgs t)))
+                    then return (TAst.funcRetType t, TAst.TCall f args')
+                    else error $ "Type mismatch in call to " ++ show f
+            Nothing -> error $ "Function " ++ show f ++ " not in scope. Available functions: " ++ show (map TAst.funcName (funcs ctx))
+
+convertExpr (Ast.Index arr idx) = do
+    arr' <- convertExpr arr
+    idx' <- convertExpr idx
+    case fst arr' of
+        ArrayType t -> if fst idx' == IntType
+            then return (t, TAst.TIndex arr' idx')
+            else error $ "Index must be an integer: " ++ show idx
+        _ -> error $ "Indexing non-array: " ++ show arr
+
+convertExpr (Ast.Cast t e) = do
+    e' <- convertExpr e
+    return (t, TAst.TCast t e')
+
+convertExpr (Ast.Sizeof t) = return (IntType, TAst.TSizeof t)
+
+convertExpr (Ast.Member e (Id m)) = do
+    e' <- convertExpr e
+    case fst e' of
+        StructType name -> do
+            ctx <- get
+            case find (\(Struct n _) -> n == name) (structs ctx) of
+                Just (Struct _ binds) -> case find (\(Bind n t) -> n == m) binds of
+                    Just (Bind _ t) -> return (t, TAst.TMember e' m)
+                    Nothing -> error $ "Field " ++ show m ++ " not in struct " ++ show name
+                Nothing -> error $ "Struct " ++ show name ++ " not in scope"
+        _ -> error $ "Member access on non-struct " ++ show e
+
+convertExpr (Ast.StructInit name fields) = do
+    ctx <- get
+    case find (\(Struct n _) -> n == name) (structs ctx) of
+        Just (Struct _ binds) -> do
+            fields' <- mapM (\(n, e) -> do
+                e' <- convertExpr e
+                case find (\(Bind n' t) -> n == n') binds of
+                    Just (Bind _ t) -> if fst e' == t
+                        then return (n, e')
+                        else error $ "Type mismatch in struct initialization: " ++ show e
+                    Nothing -> error $ "Field " ++ show n ++ " not in struct " ++ show name) fields
+                
+            return (StructType name, TAst.TStructInit name fields')
+        Nothing -> error $ "Struct " ++ show name ++ " not in scope"
+
+convertExpr e = error $ "Invalid or Unimplemented conversion for expression " ++ show e
+
+-- Ensure that the types of the left and right expressions are the same
+-- and return the type of the result
+arithOp :: MonadState Ctx m => Ast.BinOp -> Ast.Expr -> Ast.Expr -> m TAst.TExpr
+arithOp o l r = do
+    l' <- convertExpr l
+    r' <- convertExpr r
+    if fst l' == fst r'
+        then return (fst l', TAst.TBinOp o l' r')
+        else error $ "Type mismatch: " ++ show l ++ " and " ++ show r
+
+-- Ensure that the types of the left and right expressions are the same
+-- and return a boolean type
+compOp :: MonadState Ctx m => Ast.BinOp -> Ast.Expr -> Ast.Expr -> m TAst.TExpr
+compOp o l r = do
+    l' <- convertExpr l
+    r' <- convertExpr r
+    if fst l' == fst r'
+        then return (BoolType, TAst.TBinOp o l' r')
+        else error $ "Type mismatch: " ++ show l ++ " and " ++ show r
+
+-- Ensure that the types of both expressions are boolean
+-- and return a boolean type
+boolOp :: MonadState Ctx m => Ast.BinOp -> Ast.Expr -> Ast.Expr -> m TAst.TExpr
+boolOp o l r = do
+    l' <- convertExpr l
+    r' <- convertExpr r
+    if fst l' == fst r' && fst l' == BoolType
+        then return (BoolType, TAst.TBinOp o l' r')
+        else error $ "Type mismatch: " ++ show l ++ " and " ++ show r
+
+bitOp :: MonadState Ctx m => Ast.BinOp -> Ast.Expr -> Ast.Expr -> m TAst.TExpr
+bitOp o l r = do error $ "Bit operations not implemented"
+
+shiftOp :: MonadState Ctx m => Ast.BinOp -> Ast.Expr -> Ast.Expr -> m TAst.TExpr
+shiftOp o l r = do error $ "Shift operations not implemented"
diff --git a/windows12.cabal b/windows12.cabal
index f274cd5..35720ce 100644
--- a/windows12.cabal
+++ b/windows12.cabal
@@ -70,6 +70,7 @@ executable windows12
         Windows12.Parser
         Windows12.CodeGen
         Windows12.TAst
+        Windows12.Semant
 
     -- LANGUAGE extensions used by modules in this package.
     -- other-extensions: