A critical factor in cell-to-cell interactions is the presence in the cell membrane of highly specific ion channels controlled by specific receptors that are bound to, and activated by, a gamut of external hormones and neurotransmitters. Through both this action on ion channels, and action on other membrane components (such as G-proteins), extracellular signals alter intracellular events, usually through the mediation of second messengers, and so provide the basis for the transduction mechanism connecting extracellular signals with intracellular effectors. This volume deals with the various ways that such membrane function is controlled.