Adrenergic receptors are key targets within the autonomic nervous system, regulating a wide variety of physiological processes. The ability to modify adrenergic receptor expression patterns in vivo has added a powerful new tool to the functional analysis of these receptors. Modification of adrenergic receptor gene expression by overexpression, genetic ablation, or site-specific mutation has added new insight to models of receptor coupling behavior, pharmacology, and subtype-specific physiological function. This review highlights some of the recent advances resulting from such genetic approaches to the study of adrenergic receptors.