β-Adrenergic receptors (β-ARs) are members of the superfamily of G protein-coupled receptors that are stimulated by the catecholamines epinephrine and norepinephine (1). As part of the sympathetic nervous system, β-ARs have important roles in cardiovascular, respiratory, metabolic, central nervous system, and reproductive functions. Mice lacking one or more of the three β-AR subtype genes (β1, β2, and β3) have been generated to elucidate the physiological role of individual subtypes. Moreover, cells and tissues extracted from these mice have been utilized as tools to understand the molecular and cellular basis of subtype-specific receptor function. These studies are summarized in this chapter.