Functional adrenal tumors causing hyperadrenocorticism occur in only 15% to 20% of dogs with naturally occurring hyperadrenocorticism. The treatment of choice for adrenal tumors is surgery, but medical treatment may be pursued if indicated. This paper focuses on the diagnosis and treatment of a 10 year old mixed breed dog with a functional adrenal tumor and subsequent discussion of the causes of naturally occurring hyperadrenocorticism. Functional adrenal tumors and pituitary-dependent hyperadrenocorticism are discussed from the clinical signs to diganosis to treatments. Histopathological criteria to differentiate an adrenocortical adenoma from an adenocarcinoma are also reviewed.