CT is the imaging procedure of choice for detecting adrenal masses. In patients with biochemical evidence of an adrenal endocrine syndrome, CT can detect or exclude an adrenal mass in a high percentage of cases. Radionuclide scintigraphy is a useful adjunct in selected cases to characterize an adrenal mass as functional cortical (NP-59) or medullary (MIBG) tissue. In this article, the spectrum of adrenal imaging findings in patients with Cushing's syndrome, Conn's syndrome (primary aldosteronism), and pheochromocytoma is described and illustrated. In patients without an adrenal endocrine syndrome, an adrenal mass is detected on CT as an incidental finding or during a search for metastatic disease. Although pathognomonic findings of adrenal hemorrhage or myelolipoma are occasionally demonstrated, most adrenal masses have nonspecific morphological CT features. Differentiation of common benign adenomas from nonadenomatous adrenal masses, including metastases, remains an important clinical problem. This article reviews the current status, advantages, and limitations of the following methods to characterize an adrenal mass: (1) percutaneous adrenal biopsy, (2) NP-59 scintigraphy, (3) unenhanced CT densitometry, and (4) opposed-phase chemical shift MRI.