A system for the closed transfer of fluids that provides substantially leak-proof sealing and pressure equalization during engagement of a cannula with a vial, during transfer of a substance from a vial chamber to a barrel chamber via the cannula, and during disengagement of the cannula from the vial is disclosed. The leak-proof sealing of the system substantially prevents leakage of both air and liquid during use of the system. The system of the present disclosure also permits pressure equalization between a vial and the system when the system is attached to the vial. The system is compatible with a needle and syringe assembly for accessing a medication contained within a vial for administering the medication to a patient.