Carcinoma in situ (CIS) of the urinary bladder is defined as a flat lesion comprising of cytologically malignant cells which may involve either full or partial thickness of the urothelium. De novo CIS constitutes less than 3% of all urothelial neoplasms; however, CIS detected concurrently or secondarily during follow-up of urothelial carcinoma constitutes 45% and 90%, respectively, of bladder cancer. CIS is noted predominantly in male smokers in the sixth or seventh decade. Patients may present with dysuria, nocturia, and urinary frequency and urgency with microscopic hematuria. Cystoscopic findings may range from unremarkable to erythema or edema. Urine cytology is an important diagnostic tool. Cellular anaplasia, loss of polarity, discohesion, nuclear enlargement, hyperchromasia, pleomorphism, and atypical mitoses are the histopathologic hallmarks of CIS. Extensive denud ation of the urothelium, monomorphic appearance of the neoplastic cells, inflammatory atypia, radiation induced nuclear smudging, multinucleation, and pagetoid spread of CIS may cause diagnostic difficulties. Together with clinical and morphologic correlation, immunostaining with CK 20, p53 (full thickness), and CD44 (absence of staining) may help accurately diagnose CIS. Fluorescent in situ hybridization analysis of voided urine for amplification of chromosomes 3, 7, and 17 and deletion of 9p has high sensitivity and specificity for diagnosing CIS in surveillance cases. Several other molecular markers, such as NMP 22 and BTA, are under evaluation or used variably in clinical pathology. Intravesical bacillus Calmette-Guerin (BCG) instillation is considered the preferred treatment, with radical cystectomy being offered to refractory cases. Chemotherapy, α-interferon, and photodynamic therapy are other modalities that can be considered in BCG-refractory cases. Multifocality, involvement of prostatic urethra, and response to BCG remain the most important prognostic factors, although newer molecular markers are being evaluated for this entity. Patient outcome varies based on whether it is de novo development or diagnosed secondary to prior or concomitant papillary bladder cancer. From a clinical perspective, the principal determinants of outcome are extent of disease, involvement of prostatic urethra, response to therapy, and time to recurrence.