The development of composites based on conductive phases dispersed in polymeric matrices has led to important advances in analytical electrochemistry, particularly in sensor devices. These new materials combine the electrical properties of graphite with the ease of processing of plastics (epoxy, methacrylate, Teflon, etc.) and show attractive electrochemical, physical, mechanical and economical features compared to the classic conductors (gold, platinum, graphite, etc.). The properties of these composites are described, along with their application to the construction of conductometric, potentiometric and amperometric sensors. The chemical modification of the composites by blending fillers that improve the analytical characteristics of the resulting sensors is discussed, particularly for the case of amperometric devices.