Adrenocortical insufficiency causes difficulty in diagnosis and morbidity out of proportion to its rarity, because of the non-specific, multi-system nature of the clinical features. Most of these are due to cortisol deficiency. Prominent features are well-known ones such as weight loss and asthenia, and hypoglycaemia. Less prominent in recent accounts are those due to failure of cellular sodium export and to vasopressin excess, which are frequent and clinically significant. For this reason, the clinical features of isolated ACTH deficiency, isolated glucocorticoid deficiency and Addison's disease overlap greatly. In addition, cortisol deficiency has secondary endocrine effects, e.g. glucocorticoid-reversible hypothyroidism, hyperprolactinaemia and hypercalcaemia. Further overlap between the various steroid insufficiency syndromes occurs because of the association of various organ-specific autoimmune endocrinopathies with Addison's disease. Over 80% of Addison's disease is of the autoimmune type, though almost any systemic destructive process can cause similar steroid insufficiency. Demonstration of adrenal insufficiency requires various combinations of tetracosactrin adrenal stimulation tests, and hypoglycaemia or equivalent tests, if the cause is ACTH deficiency but the correct test can only be chosen to suit a firm clinical diagnosis. The treatment of adrenocortical insufficiency is described.