The structural and electrical properties of the epitaxial CdS-CdTe heterostructures are reviewed. Both CdS epitaxial layers grown on CdTe substrates and CdTe layers on CdS substrates are discussed. In both systems epitaxy appears to be dependent on the substrate polarity, with the best epitaxy occurring on the non-metal polar surfaces. Current transport is controlled by a multi-step tunneling/recombination process in both types of structure. The photovoltaic characteristics of CdS layers on CdTe substrates are dominated by the high bulk resistance of the CdTe. The photovoltaic properties of the CdTe layers on CdS substrates are less affected by substrate resistance (since the CdS substrates are generally more conducting) but are limited by greater junction losses resulting in lower open circuit voltages.