Pituitary adenomas originating in adenohypophysial cells represent the most common neoplasms of the sella turcica. The pathologist's goal is the optimal diagnosis and classification of pituitary adenomas. Lack of clinicopathological correlations in the past classification of pituitary adenomas, which was based on the tinctorial properties of adenoma cells, limited the importance of histological diagnosis. Morphologic separation of pituitary cells by electron microscopy provided fundamental knowledge to classify pituitary adenomas. Immunohistochemistry represents the gold standard of the current classification. Combined morphologic and immunohistochemical diagnostic approaches resulted in the clinicopathologic classification of pituitary adenomas. The WHO classification of 2004 is based on morphologic features and takes into consideration findings from imaging procedures and clinical symptoms. Morphologic characterization of pituitary tumors and correlation of the hormone product with hormone secretion provides the clinician with useful information. In addition, the utility of tumor markers offers objective information in managing the patient and predicting responses to specific treatment. The Ki-67 labeling index (LI) is widely used for it correlates with invasiveness and probably prognosis. Adenomas showing increased ( >3%) LI and extensive p53 immunoreactivity should be termed "atypical adenomas" suggesting aggressive potential or malignant transformation. Morphologic separation of adenoma from carcinoma is not feasible. The term pituitary carcinoma should be exclusively applied when cerebrospinal and/or systemic metastases are definite.