The morphological features of the adult human adrenal cortex are described with particular respect to changes induced by alterations in function of the hypothalamo-pituitary axis. The occurrence of nodules in the normal and hyperplastic cortex (Cushing's and Conn's syndromes) is discussed in relation to the diagnostic problems that they still pose. Explanations based on the normal mechanisms of functional zonation in the cortex are provided for the different cell types which comprise cortical neoplasms associated with various syndromes of hypercorticalism (Cushing's, adrenogenital and Conn's syndromes), together with morphological and functional criteria to distinguish adenomas from carcinomas.