Abstract: A closed system transfer device includes a first adaptor and a second adaptor that attach with one another in a coupled state to form a sealed fluid passage between two reservoirs. The first adaptor can be attached to a first reservoir, and the second adaptor can be attached to a second reservoir. One or both adaptors can include a septum that engages another septum during coupling and decoupling to prevent liquid or vapor from being released to the environment. The device can also include a vent line that equalizes pressure between the two reservoirs. The vent line can include at least one filter to filter gas in device. In addition, or in the alternative, the vent line can be connected in fluid communication with an inflatable membrane that stores gas inside the device and assists in equalizing pressure.