With the increasing use of abdominal cross-sectional imaging, incidental adrenal masses are being detected more often. The important clinical question is whether these lesions are benign adenomas or malignant primary or secondary masses. Benign adrenal masses such as lipid-rich adenomas, myelolipomas, adrenal cysts and adrenal haemorrhage have pathognomonic cross-sectional imaging appearances. However, there remains a significant overlap between imaging features of some lipid-poor adenomas and malignant lesions. The nature of incidentally detected adrenal masses can be determined with a high degree of accuracy using computed tomography (CT) and magnetic resonance imaging (MRI) alone. Positron emission tomography (PET) is also increasingly used in clinical practice in characterizing incidentally detected lesions. We review the performance of the established and new techniques in CT, MRI and PET that can be used to distinguish benign adenomas and malignant lesions of the adrenal gland.