Carcinoma in situ is a high-grade and aggressive manifestation of transitional-cell carcinoma of the bladder that has a highly variable course. The treatment of CIS has undergone dramatic changes since this malignancy was first recognized. While cystectomy was once recommended as the initial treatment of choice, recognition of the highly variable prognosis and the uniformly high response rate to intravesical BCG has prompted a more conservative approach to management. While it is recommended that patients be offered the option of radical cystectomy, data do not currently exist to confirm that cystectomy provides a superior survival or quality of life compared with an initial trial of BCG immunotherapy followed by salvage cystectomy if needed. With current optimal BCG immunotherapy regimens, which consist of a 6-week course of BCG followed by three weekly instillations at 3 months, 6 months, and every 6 months for 3 years, the complete response rate is 82%; and it is estimated that more than 75% of patients having a complete response will remain continuously disease free for 5 or more years. Patients who fail BCG immunotherapy without evidence of progression may yet be candidates for intravesical chemotherapy, photodynamic therapy, or alternative immunotherapies such as alpha-2b interferon, bropirimine, or keyhole limpet hemocyanin.