There is provided a spill proof cup assembly having a cap (24) with at least one spout outlet (26), a cup (10) with an upper open poriton (22) adapted to securely and sealing receive the cap (24), anda thermoplastic elastomer, or similar elastomeric material, co-molded bottom portion (40) with a vent disposed therein for allowing air to enter the cup as fluid exits through the spout outlet. The result is a one-way flow of air.