Glucocorticoids mainly act through binding to cytosolic receptors that translocate to the nucleus after ligand binding, and dimerize to affect gene transcription in multiple fashions. The liganded receptors may interact with DNA at specific glucocorticoid responsive-elements, may physically hinder the ability of other transcription-regulating proteins to interact with their own DNA response-elements, and may form intranuclear complexes with the transcription factor c-jun, thus changing the number of c-jun/c-fos heterodimers that bind at AP-1 sites. By these, and perhaps other, mechanisms, physiologic concentrations of glucocorticoids regulate normal tissue metabolism, and supraphysiologic concentrations cause Cushing's syndrome. Cushing's syndrome leaves virtually no body tissue untouched. Left untreated, it results in progressive adiposity, myopathy, dermopathy (atrophy, stria, purpura, and hirsutism), psychopathy, glucose intolerance, hypercholesterolemia, hypertension, atherosclerosis, immunosuppression, and, ultimately, death. The physiology underlying each of these effects of hypercortisolism has been reviewed. The differences in the presentation of Cushing's syndrome in children and adults have also been discussed. The goal of the clinician must be to identify individuals with Cushing's syndrome as early in the course of the disease as possible so as to avoid the devastating complications that result from prolonged hypercortisolism. In patients for whom screening tests are equivocal, or only intermittently elevated, it may be necessary to re-evaluate the patient over time to establish that the patient has hypercortisolism. Some clinical guidelines for which patients to screen for hypercortisolism have been presented. Once hypercortisolism is established, patients with mild hypercortisolism (urine free cortisol less than four-fold above the upper limit of normal) should undergo tests to differentiate true Cushing's syndrome from a pseudo-Cushing state.