Interfaces play a crucial role in all solid-state electronic devices. In organic transistors there are two kinds of interfaces: between the gate dielectric and the semiconductor layer, and between the semiconductor and the source and drain contacts. The former is the very place where charge transport in the conducting channel occurs, while the latter governs charge injection and removal to and from the external circuit. Because of the importance of the insulator—semiconductor interface, the choice of the dielectric is at least as critical as that of the semiconductor. On the other hand, charge injection is currently considered as at least as crucial as charge transport in the organic semiconductor. In this chapter, the relevant issues associated with the interfaces are developed. Particular attention is paid to the characterization of the interfaces and to models developed to understand and improve their properties.