The adenylyl cyclases are a family of enzymes that play important roles in areas such as cancer biology, endocrinology, cardiovascular physiology and neurobiology. This work describes research concerning the cloning and purification of the adenylyl cyclases and the unique properties of each adenylyl cyclase. Specific topics covered include: mechanisms for receptor coupling to adenylyl cyclases through the G coupling proteins; regulation of the adenylyl cyclases by protein kinases; Ca2+; and membrane potential. Evidence indicating a role for the Ca2+ sensitive adenylyl cyclases in neuroplasticity is also reviewed.