The classification of epithelial tumors of the kidney has undergone considerable change in the last two decades. Systems based on cytoplasmic characteristics and cytogenetic analysis have expanded our understanding of this group of tumors. These new, nontraditional systems have led to the development of a more clinically significant pathological classification (1 ,2 ). Although many questions remain unanswered and debate continues concerning the validity of these proposals, research studies on epithelial neoplasms of the kidney must take these advances into consideration. Scientific studies of any type should incorporate information regarding the type of tumor(s) included in the study group. This chapter briefly reviews the accepted subtypes of renal epithelial neoplasms, with a focus on the morphological features that distinguish them.