Corticosteroids are an essential component of the body's homeostatic system. In common with other such systems, this implies that corticosteroid levels in blood and, more importantly, in the tissues remain within an optimal range. It also implies that this range may vary according to circumstance. Lack of corticosteroids, such as untreated Addison's disease, can be fatal in humans. In this review, we are principally concerned with excess or disturbed patterns of circulating corticosteroids in the longer or shorter term, and the effects they have on the brain.