The majority of adrenal masses are discovered incidentally with ultrasound imaging and are benign adrenal incidentalomas (AI). CT scan and MRI are the two most important imaging modalities used for diagnosing an adrenal mass and are often sufficient to discriminate benign from malignant lesions. Tumor size alone is not a reliable determinant of malignancy. In fact, the prevalence of malignancy in patients with adrenal masses of ≥4 cm in diameter is maximally 30%. CT and MRI are also important in the diagnosis of pheochromocytoma and hormone-secreting lesions. CT with contrast medium or unenhanced CT and 18F-fluorodeoxyglucose-positron emission tomography (18F FDG-PET/CT) are used as second-line imaging modalities with a sensitivity of 87% and a specificity of 84%. 18F FDG-PET/CT may accurately differentiate benign AI from malignant tumors and adrenal metastases with a sensitivity of 100% and a specificity of 94%, according to some authors. The diagnostic accuracy rate is 96%. Percutaneous image-guided biopsy is indicated only in very selected cases of indetermined adrenal nodules and only after hormone tests have been performed to exclude pheochromocytoma. Adrenal vein sampling (AVS) is the most accurate diagnostic modality for the diagnosis of primary aldosteronism and should be performed in all patients older than 35 years of age, even if an adenoma has been detected incidentally by CT or MRI.