Adrenergic receptors are cell surface glycoproteins that recognize and selectively bind the catecholamines, norepinephrine and epinephrine, which are released from sympathetic nerve endings and the adrenal medulla. By transducing the external catecholamine stimulus into an intracellular signal, these receptors mediate the actions of the sympathetic nervous system, including a variety of responses such as arteriolar smooth muscle contraction and cardiac contraction, while are critically involved in cardiac function and blood pressure homeostasis. Activation or blockade of these receptors is, thus, a major therapeutic approach for the management of a number of cardiovascular disorders, including hypertension, angina pectoris, and cardiac arrhythmias. Additionally, alterations in these receptors or in their coupled intracellular effectors may contribute to or underlie the pathogenesis of cardiovascular diseases such as cardiac hypertrophy, congestive heart failure, and hypertension. Over the past few years, major insights have been gained into the physiology and pharmacology of these receptors, particularly in relationship to the molecular mechanisms involved in their ability to mediate "transmembrane signalling." These insights, which will be reviewed here, have resulted from the isolation and characterization of adrenergic receptors, from the determination of their deduced primary and secondary structures, and from structure-function studies using classical biochemistry and pharmacology as well as recombinant DNA techniques.