This document summarizes crosshole shear wave testing. It discusses that crosshole testing involves lowering a source and receiver down separate boreholes to measure seismic wave velocities between holes. Common applications include bridge/foundation analysis, material testing, soil/rock mechanics, and earthquake/liquefaction analysis. Key considerations are distinguishing refracted from direct energy arrivals, difficulty measuring shear waves, and deviations from vertical/straight boreholes leading to errors. Benefits are direct velocity measurements with depth, while limitations include inability to detect lower velocity layers and need for source power matching desired depth.