In this chapter the so-called Vertical Seismic Profile (VSP) is discussed: the seismic sensors are in the borehole while the seismic source is still at the surface. First, different types of VSP’s are discussed. Next, it is explained how to obtain a reflectivity profile from raw VSP data. The result serves a calibration for the surface seismic image: in VSP’s we know depth and time, so the velocity is much better known than from surface seismic.