The first insight into the etiology of transitional cell carcinomas of the urinary bladder began with the observation of an increased incidence associated with industrial development. Workers in the aniline dye

second most common urologic malignancy. In addition, these tumors are one of the best understood genitourinary neoplasms with a well defined etiology, natural history, tumor biology, treatment options and outcome. This level of understanding arises as a consequence of multiple factors and represents a convergence of knowledge from diverse scientific disciplines. Insight provided by these disciplines, coupled with unique features of this neoplasm which make it assessable for detection, monitoring and treatment, combine to make this disease a model system for modern oncology. The intent of this review is to provide the reader an overview of our current understanding of this tumor from the standpoint of its molecular biology as related to tumor development and progression.