Seismic reflection profiling across strike-slip faults has revealed a distinct variation in deep crustal structure. Profiles across the San Andreas, Great Glen, and Bray faults depict a near-vertical, reflection-free zone that appears to truncate and in some cases offset reflections interpreted to have originated from the base of the crust. A near-vertical, crustal-penetrating fault zone is suggested for these continental transform faults. In contrast, reflection profiles across intra-plate strike-slip fault systems such as the northern Walker Lane and Garlock faults suggest that they are decoupled in the middle crust by subhorizontal detachments. These detachments are inferred from reflections that are traceable below the surface faults without disruption. Such detachments in the middle crust may act to ease the rotation of upper-crustal blocks adjacent to strike-slip fault zones, as observed in paleomagnetic studies. Furthermore, moderately dipping reflections, interpreted to be from the fault zone, project to the surface traces of the northern Walker Lane faults and the frontal faults of the Wichita Uplift, indicating that these strike-slip faults with large components of dip-slip displacement are not vertical but dip moderately into the mid-crust. Although the available data sets are still few, they suggest that there is a primary variation in fault geometry that is related to the different behavior of transform versus intra-plate strike-slip fault systems.