Exfoliative cytology is a relatively sensitive, highly specific technique for the detection and diagnosis of bladder cancer, and for monitoring conservative treatment. Cytologic diagnosis is based on cellular changes in DNA content, chromatin structure, and metabolic activities which are characteristic of cancer cells and can be recognized in desquamated isolated cells or cell clusters. The cytologic techniques are most effective in identifying low stage flat or papillary carcinoma, but not cytologically benign papillomas or ulcerating invasive cancers that do not easily shed cancer cells. With the growing number of pathologists who, in recent years, have become skilled cytopathologists, urinary tract cytology has developed into an essential diagnostic tool for the urologist.