Pituitary hyperplasia is rare, difficult to diagnose and sometimes controversial. The hyperplasia could be physiologic which is usually reversible, or pathologic which varies in presentation from incidental to tumor like lesion with and without hormonal disturbance. Any pituitary cell is capable of undergoing hyperplasia in the presence of the right stimuli. In this article we summarize the various pathologic and morphologic features of each subtype of pituitary hyperplasia, give an account of the molecular, hormonal and cellular basis of this condition and outline its clinical significance, differential diagnosis and prognosis.