Adrenal insufficiency is an endocrine disorder with a low prevalence, but it can potentially be fatal if not treated. It is caused by a deficit in cortisol production by the adrenal glands, a deficit in pituitary ACTH production, or can occur after abrupt suppression of glucocorticoid intake. Hemodynamic collapse is the usual clinical presentation in an acute adrenal crisis. The diagnosis entails demonstrating a cortisol deficiency, for which dynamic tests are available to evaluate the adrenal reserve. Knowing the method used in the biochemical determination of cortisol is of interest in order to establish appropriate reference ranges. Restoration of volemia and the use of intravenous glucocorticoids constitute the initial management of an acute crisis and must never be delayed when there is clinical suspicion. In chronic forms, oral glucocorticoid treatment at physiological doses is started. Mineralocorticoid treatment is indicated only for the primary etiology.