Detonation diamond nanoparticles (nanodiamond) show interesting and unexpected redox behaviour and have been incorporated into a variety of electrochemical technologies, e.g., sensors, biosensors, fuel cell and supercapacitor electrodes. This redox behaviour is unexpected because the particles are undoped and not conductive. The surfaces of the particles are highly unsaturated and oxidised with an array of surface functional groups and it is believed that the origin of nanodiamond redox activity can be attributed to this surface chemistry. This chapter presents the current understanding of the nature and origin of the electrochemical response of nanodiamond particles and how the particles interact with electrodes and redox species in solution. Additionally, a survey of the current literature concerning the application of nanodiamond in electrochemistry is included, along with a brief discussion of the redox response of other types of diamond nanoparticle, such as high temperature high pressure (HTHP) diamond.