The diagnosis and treatment of hypoadrenocorticism can be one of the greatest challenges faced by veterinary practitioners, as Addison's disease may have many faces and many presentations. Although the disease is most often diagnosed in dogs, cats may also suffer from Addison's disease. The practitioner must have a high index of suspicion to make a diagnosis of hypoadrenocorticism. This index of suspicion is based on knowledge of the common signalment, history, physical examination, and laboratory findings. Diagnosis of hypoadrenocorticism is supported by appropriate choice of diagnostic endocrine tests that are described in detail in this article. Once a diagnosis of hypoadrenocorticism has been made, expedient treatment is of foremost concern. Timely treatment using fluids, corticosteroids, and supportive care will ensure a successful outcome; the emergency treatment of Addison's is covered briefly in this article and fully in another article in this issue. The purpose of this review was to describe the clinical diagnosis and chronic treatment of hypoadrenocorticism in dogs and cats.