Polarized electron beams for linear colliders*

Longitudinally polarized electron beams for high energy collisions provide a sensitive way to explore the electroweak process as well as an effective means to measure spin dependent properties of particles. Once created, such beams are readily accelerated by linacs without loss of polarization, although emittance damping rings present potential hazards. The essential elements of a collider necessary for the utilization of polarized electrons are described. The key element is the polarized electron source as illustrated by the SLC which now operates with Pe 80% in the 50 GeV linac. Possible improvements for future colliders are discussed.