In general, almost any generation Orbit seat (Car Seats, Stroller Seats, and Bassinets) is compatible with any generation Orbit base (Car Seat Base, Stroller Base, and Rocker Base). The main exception involves first-generation (G1) seats and bases. The Orbit Baby Sidekick is compatible with the G5 Travel System, as well as previous generation G2 and G3 Stroller Frames