The size-dependent optical properties of semiconductor nanocrystals have been extensively studied, as described elsewhere in this volume. In this chapter, we concentrate instead on the electrical properties of nanocrystals. We show how electrons can be injected into nanocrystals, transported between nanocrystals and transferred between nanocrystals and organic molecules. We then describe the potential applications of nanocrystals in electronic and optoelectronic devices. We attempt to give an overview of the field, although we cannot mention all of the articles which have been published in this diverse and expanding area. We draw many of our examples from our own work and from the work of those other groups with which we happen to be most familiar. We restrict ourselves to chemically synthesized semiconductor nanocrystals and concentrate mostly on II-VI nanocrystals such as CdSe in the size regime of less than 10 nm, where quantum confinement effects are important.