This review discusses interfacial layers in organic photovoltaic devices. The first part of the review focuses on the hole extraction layer, which is located between a positive electrode and an organic photoactive material. Strategies to improve hole extraction from the photoactive layer include incorporation of several different types of hole extraction layers, such as conductive polymeric materials, self-assembled molecules and metal oxides, as well as surface treatment of the positive electrodes and the conductive polymeric layers. In the second part, we review recent research on interlayers that are located between a negative electrode and a photoactive layer to efficiently extract electrons from the active layer. These materials include titanium oxides, metal fluorides and other organic layers.