Adrenal insufficiency is broadly defined as an inability to produce an appropriate amount of adrenocortical steroids. More specifically, adrenal insufficiency may be classified into two larger categories based on underlying pathophysiology: primary adrenal insufficiency (also referred to as Addison’s disease), resulting in a deficiency of both glucocorticoids and mineralocorticoids, and secondary adrenal insufficiency, resulting from a deficiency in adrenocorticotropic hormone (ACTH) secretion and consequent impaired adrenal production of cortisol. These two forms of adrenal insufficiency have unique and shared features. In order to understand the differences and similarities of each form of adrenal insufficiency and appreciate how the diagnosis and optimal management of adrenal insufficiency should be made, this chapter will first briefly review the physiology of the adrenal cortex and then detail the treatment of adrenocortical deficiency. This chapter will not cover the adrenal medulla, which synthesizes and secretes catecholamines.