Critically ill patients at some stage may develop adrenal insufficiency (AI). This article reviews the mechanisms, diagnosis criteria, consequences, and treatment of AI in various ICU conditions. Glucocorticoid insufficiency may be related to a decrease in glucocorticoid synthesis (ie, adrenal insufficiency) or to a reduced delivery of glucocorticoid to target tissues and cells. Diagnosis relies on clinical suspicion and ACTH test results. The length of cortisol replacement therapy should be at least 7 days and the adjunction of fludrocortisone is recommended.