Classsification of thymomas is difficult and controversial. The traditional approach to classification of thymomas is based on predominant cell type proposed by Bernatz in 1961. In 1985, Marino and Müller-Hermelink developed a new classification of thymic epithelial neoplasms based on the resemblance of the neoplastic epithelial cells, both morphologic and functional, to normal thymic cortical and medullary epithelial cells. The last classification proposed by Suster and Moran in 1999 is based on the premise that primary thymic epithelial neoplasms correspond to a single family of lesions showing a variable spectrum of differentiation. In this study, all classifications of thymomas have been revieved.