Isolation valves for selectively sealing a first region from a second region. A gate valve can include a housing which defines a channel between the first and second regions. The valve includes a gate, located in the housing, and displaceable between a stowed position and a deployed position. When the gate is in the stowed position, communication is permitted between the first and second regions. When the gate is in the deployed position, the gate spans the channel and can be controlled to isolate the first and second regions. The valves can be used, for example, in connection with systems for processing large glass substrates. The valves are particularly useful for isolating long rectangular openings, such as the openings in substrate processing chambers. Isolating processing chambers or load lock chambers from one another, for example, in a linear system, is facilitated.