The diagnosis of adrenocortical disease depends on accurate and precise laboratory measurements. The optimal application of the available tests is an evolving process. This chapter concentrates on those tests that the author believes are most useful. The discussion focuses on three categories of test: tests used to document hypercortisolism or hypocortisolism; tests used for the differential diagnosis of hypercortisolism or hypocortisolism; and tests used to localize disease. Further details concerning the rationale and interpretation of adrenal function tests are found in Chapter 14, Chapter 17, Chapter 75, Chapter 76, Chapter 77, Chapter 78, Chapter 79, Chapter 80, Chapter 81, Chapter 82, Chapter 83, Chapter 237 and Chapter 241.