Abstract In 1855, Thomas Addison published a monograph in which he described 11 patients with adrenal disease (Addison 1855; Jeffcoate 2005). He emphasized their “general languor” and, in keeping with his interest in dermatological conditions, the “peculiar change” in skin color. This illness, caused by destruction of the adrenal cortex, has come to be known as Addison disease (AD). In this chapter, we begin by outlining the basic biology pertinent to an understanding of AD, followed by an outline of the clinical features of the illness and the etiological factors to be considered in children and adolescents presenting with AD. We then discuss the neuropsychiatric manifestations of AD and the mechanisms that can give rise to these. The adrenal glands, which sit on the upper poles of each kidney, are comprised of a number of distinct subregions, each of which is specialized for the synthesis and release of a particular hormone. The inner zona fasciculata of the adrenal cortex is responsible for the production and secretion of the glucocorticoid, cortisol, which participates in the hypothalamo-pituitary-adrenal (HPA) axis. The outer layer of the adrenal cortex, called the zona glomerulosa, produces the mineralocorticoid aldosterone, which is part of the renin-angiotensin system. The zona reticulata secretes the androgen dehydroepiandrosterone (DHEA), which is partially controlled by adrenocorticotropic hormone (ACTH) released from the pituitary. All three of these steroid hormones are affected in AD. The clinical signs and symptoms have traditionally been attributed to the deficiencies of cortisol and aldosterone, although recent evidence suggests that reduced levels of DHEA might play a role in the development of mood disturbances and the general loss of well-being in women with AD. The adrenal medulla, which secretes epinephrine and norepinephrine in response to stimulation from the sympathetic nervous system, is anatomically distinct from the cortex and not thought to be affected in AD (Hunt et al. 2000; Oelkers 1999). There is, however, compelling evidence of “cross-talk” or bidirectional interaction between these two parts of the adrenal gland (Bornstein and Chrousos 1999), both of which are critical components of the stress response.