In the past several years, with the advent of the patch clamp technique, the field of leukocyte electrophysiology has grown considerably. With the exceptions of the neutrophil and the eosinophil, electrophysiological and biochemical techniques have been used to characterize a number of types of voltage- or ligand-gated ionic channels in the different classes of leukocytes. This article reviews each of the ionic channels described in leukocytes and their functional relevance. It should be emphasized that this is by no means a final listing of the ionic channels in leukocytes, but merely a summary of the studies to date. It is highly likely that there are many other ionic channels in leukocytes with important functional implications that have not yet been discovered.