The state of stress and style of crustal deformation along weak transform plate boundaries is discussed in the context of available data and simple analytical models. Appreciable evidence indicates that while the frictional strength of the upper crust is high (in general accordance with simple faulting theory and laboratory-derived coefficients of friction), the frictional resistance to motion along transform plate boundaries is extremely low. These conditions require that horizontal principal stresses must be oriented approximately parallel and perpendicular to the transform-fault zone to minimize the shear stresses acting parallel to the transform. Along plate margins that must accommodate relative convergence (transpressive margins), a pattern of near-fault-normal compression and fault-normal crustal shortening is expected. Along divergent plate margins (transtensional margins), extension is expected to occur perpendicular to the transform, as the direction of minimum horizontal principal stress is expected to be nearly perpendicular to it. These patterns of stress and deformation can be observed along a number of transform faults around the world.