Endogenous Cushing's syndrome is a clinical state resulting from prolonged, inappropriate exposure to excessive endogenous secretion of cortisol and hence excess circulating free cortisol, characterized by loss of normal feedback mechanisms of the hypothalamo-pituitary-adrenal axis and normal circadian rhythm of cortisol secretion. The etiology of Cushing's syndrome may be excessive ACTH secretion from the pituitary gland, ectopic ACTH/CRH secretion by non-pituitary tumor, or excessive autonomous secretion of cortisol from hyperfunctioning adrenal adenoma or carcinoma, PPNAD and MAH. Other than these broad ACTH-dependent and ACTH-independent categories, there are some unclassified variants as cyclical Cushing's syndrome, pituitary hyperplasia and subclinical Cushing's syndrome. In addition, variants of hypercorticism secondary to ectopic or aberrant expression of several G-protein-coupled receptors have been identified. Diagnosis of Cushing's syndrome must be made before any attempt at differential diagnosis, and key biochemical characteristics are: excess endogenous cortisol secretion; loss of normal feedback of hypothalamic-pituitary-adrenal axis; and disturbance of normal circadian rhythm of cortisol secretion. Biochemical diagnosis of Cushing's syndrome includes: urinary free cortisol determination, low-dose dexamethasone testing, circadian rhythm assessment, insulin tolerance test, and LDDST/CRH test. Differential diagnosis of Cushing's syndrome involves: plasma ACTH level determination, high dose dexamethasone testing, metyrapone testing, testing with CRH, testing with vasopressin or combination, and finally, bilateral simultaneous petrosal sinus sampling with CRH stimulation.