Carcinoma is situ (CIS) of the bladder is a high-grade non-invasive malignancy with a high tendency of progression and transitional cell carcinoma outside the bladder. The diagnosis is a combination of abnormal cytology and cystoscopy with biopsies. Although cytology has clear limitations in low-grade lesions, such as a low inter- and intra-observer reproducibility, high-grade lesions and CIS should be diagnosed with a high degree of sensitivity and specificity. Currently available urinary markers do not (yet) seem to match cytology. The cystoscopic diagnosis is more difficult, since flat lesions are often difficult to see. The application of fluorescence cystoscopy and resection clearly improves the detection of the number of CIS lesions per patient and also the number of patients with CIS. For treatment of CIS (maintenance) BCG remains the golden standard. BCG appears to be able to prevent or delay progression to muscle invasive disease. BCG refractory patients are at high risk for progression and cancer death, and cystectomy is the treatment of choice. Alternatives for BCG refractory CIS patients, like intravesical chemo-immunotherapy, new chemotherapeutic drugs or photo-dynamic therapy, remain highly experimental. Last but not least, the danger for CIS patients is failure to respond to therapy and a high subsequent chance of progression and cancer-specific death. Unfortunately, despite much research, this prediction is not yet possible with molecular markers in daily practice.