Adrenal insufficiency is characterised by inadequate glucocorticoid production owing to destruction of the adrenal cortex or lack of adrenocorticotropic hormone stimulation. In primary adrenal insufficiency, lack of mineralocorticoids is also a feature. Patients can present with an insidious onset of symptoms, or acutely in adrenal crisis, which requires prompt recognition and treatment. Chronic glucocorticoid therapy is the most common cause of adrenal insufficiency. The diagnosis of adrenal insufficiency is made by demonstrating low basal and/or stimulated serum cortisol and should be followed by appropriate investigations to establish the underlying aetiology. Maintenance glucocorticoid replacement is usually given as a twice or thrice daily hydrocortisone preparation. Patients with primary adrenal insufficiency also require mineralocorticoid. Regular monitoring for features of under- and over- replacement is essential during follow-up. Patient education is a key feature of management of this condition.