Pituitary adenomas should be classified by the grade of differentiation in undifferentiated (acidophil or mucoid cell type) and in highly differentiated types. Corresponding to their structural identify, the latter are separated into GH cell-, prolactin cell-, TSH cell-, or ACTH cell-adenomas. Chromophobe adenomas which are nearly ungranulated tumors, were divided into small cell- and large cell-adenomas and into oncocytic adenomas. Undifferentiated acidophil and GH cell-adenomas mostly induce an acromegaly. Undifferentiated mucoid and ACTH cell-tumors result exclusively in a hyperfunction of ACTH. The morphological correlate of a hyperprolactinemia is variable. Chromophobe adenomas of small cell type and oncocytic adenomas are on the whole endocrinologically inactive. Studies of the paraadenomatous adenohypophysis let us assume that prolactin cell- and ACTH cell-adenomas develop from hyperplasias of the specific cells. In animal experiments, pituitary adenomas can be classified into highly differentiated (prolactin cell-, TSH cell-, or ACTH cell-adenomas) and also into undifferentiated adenomas. The highly differentiated ones develop hyperplasiogenic and are monohormonally active. They are on the whole structurally identical with the human adenomas. The undifferentiated ones are the consequence of a process of dedifferentiation. They show in part a plurihormonal activity.