Human pituitary adenomas are benign neoplasms composed of hormonal adenohypophyseal cells. They arise in the sella turcica, and are characterized by a wide range of biological behavioral related not only to hormonal but also to proliferative activities. In the past, pituitary adenoma studies have mainly been devoted to histological and ultrastructural classification improved by immunohistochemical techniques. Molecular biology, cytogenetic studies, associated to experimental animal models, transgenic and knockout mice technologies, have allowed new approaches, especially concerning pathogenesis and progression. It became obvious that tumorigenesis has to be considered as a multistep event, inasmuch as different factors share numerous homologies. On the other hand, it could be supposed that the knowledge of the biological aggressiveness could led to an appropriate follow-up as well as specific therapy.