The size-dependent optical properties of semiconductor nanocrystals have been extensively studied, as described elsewhere in this book. This chapter concentrates instead on the electrical properties of nanocrystals. The processes by which electrons may be injected into nanocrystals, transported between nanocrystals and transferred between nanocrystals and organic molecules are described, and the potential applications of nanocrystals in electronic and optoelectronic devices are outlined. The focus is on chemically synthesized semiconductor nanocrystals, in particular II-VI nanocrystals such as CdSe in the size regime of less than 10 nm where quantum confinement effects are important. Since the first edition of this chapter, coauthored with David S. Ginger and published in 2003, 1 there have been many important developments. In this revised edition, the theory section has been expanded to cover 236the regime of high charge densities, and the remainder of the review has been refocused onto more recent work. Given the rapid expansion of this area of research it is not possible to mention every paper, or even every important paper, published in the field; examples are drawn from the work of my own group and of those groups with whose work I happen to be most familiar.