CT is the imaging procedure of choice for the detection of most suspected adrenal masses. But except for some patients with acute adrenal hemorrhage or fat-containing myelolipoma, the precise histologic nature of an adrenal mass is not apparent from the CT image. MIBG radionuclide scanning is useful in some patients with pheochromocytoma, whereas bilateral adrenal venous sampling for hormone assay is necessary for correct lateralization in some patients with a small aldosterone-producing adenoma. The potential value of MR imaging in the characterization of adrenal masses, especially to distinguish benign adrenal cortical adenomas from metastatic disease, is now under investigation. Currently percutaneous aspiration biopsy is still necessary to make this distinction in patients with an adrenal mass and a known extra-adrenal primary neoplasm.