Diamond and nanodiamond powders occupy a unique place as electrode materials due to their extreme properties, such as hardness, chemical inertness, thermal conductivity, optical and electric properties. Highly boron-doped diamond electrodes exhibit desirable conductivity, remarkable chemical resistance to fouling, a wide potential window, low background currents and exceptional mechanical stability. This chapter presents some of the recent work on the electrochemistry of diamond and, in particular, diamond nanopowders in their conductive and insulating forms.