This talk will address plate-boundary, strike-slip faults that traverse the entire lithosphere: The hypothesis is that the interaction of the different lithospheric layers is critical to understanding the behaviour of the system. I will present data from different field areas, including: 1) The crustal, creeping segment of the San Andreas fault; 2) Xenoliths from beneath the San Andreas fault system (Calaveras fault) and Baja California shear zone; 3) The Bogota Peninsula shear zone in New Caledonia, a complete cross-section through a transform fault within the oceanic lithospheric mantle currently exposed at the Earth’s surface. Analyses of the Bogota Peninsula shear zone indicate that the strength of the lithospheric mantle is significantly lower than predicted by experimental deformation. The only indication of high stresses occurs in microscopic deformation zones, which are interpreted as fractures (formed during earthquakes) that are subsequently deformed ductilely during post-seismic flow. The same low strength is observed in the San Andreas and Baja California xenoliths. The difficulty with trying to understand the upper crustal response to mantle flow is that it is typically confounded by the seismic cycle. The one place that this problem can be avoided is in the creeping segment of the San Andreas fault system. In this location, geodetic and structural data can be combined to provide a record of crustal deformation. We combine the strengths of the different layers within the lithosphere to propose the lithospheric feedback model for how plate boundary strike-slip faults operate. These studies show that lithospheric strength is a dynamic property—varying in space and time—in actively deforming regions.