This brief review summarizes various schemes which were introduced to classify pituitary tumors of adenohypophysial origin. Many different classifications were proposed. Pathologists prefer classifications based on the morphologic features of tumor cells. The gold standard is the light microscopic study of hematoxylin-eosin stained sections of formalin fixed and paraffin-embedded tissues. For correlation between hormone production, secretory activity and cytogenesis, immunohistochemical and transmission electron microscopic investigation is needed. We are convinced that in the future, molecular and genetic techniques will also be applied. We present here our five-tier scheme for classification of pituitary tumors which was accepted by the 'World Health Organization International Histological Classification of Tumours'. This classification takes into consideration the clinical and laboratory findings, imaging results, histologic, immunocytochemical and ultrastructural features of tumor cells. Despite several recent attempts to assess the growth rate, aggressiveness and invasiveness of pituitary tumors, more work is required to draw conclusions on their prognosis.