This article discusses some of our recent work on the origins of redox activity of undoped nanodiamond (ND) powders, as well as reviewing some properties and applications of this material. The electrochemical activity is attributed to unsaturated bonding at the ND particle surface; hence the most recent understanding of the surface chemistry of these materials is discussed. The implications of the observed redox activity, especially for use in biological applications, are highlighted, along with future avenues of research.