This chapter presents a state of the art summary of adrenal tumors relation to its biology, epidemiology, disease mechanisms, clinical signs, diagnosis, treatment and prognosis. Tumors of the adrenal glands include neoplasms affecting the adrenal cortex and those affecting the adrenal medulla. Adrenocortical tumors are essentially epithelial tumors that can be divided into adrenal cortical adenoma (ACA) and adrenal cortical carcinoma (ACC). ACA is a benign neoplasm resulting from neoplastic proliferation of adrenal cortical cells. Some ACA is functional and may produce a pure or mixed endocrine syndrome, ranging from hyperaldosteronism/Conn's syndrome, hypercortisolism/Cushing's syndrome to virilization. Diagnosis of adrenal tumors involves a medical history review, physical examination, biochemical tests, imaging studies, and pathological examination after tumor removal. ACC is an unencapsulated, brown–orange–yellow mass with variegated cut surface, calcification, and invasion of lymphatic channels/blood vessels. Functional adrenal tumors are usually treated by surgery, while nonfunctional and non-premalignant ACA is managed conservatively with long-term follow-up; surgical excision is unnecessary.