Contribution of immunotherapy in the treatment of advanced biliary tract cancer Biliary tract cancers (BTC) are rare tumors with a poor prognosis. Their treatment, at an advanced stage, relies on combinations of chemotherapies, which have a limited range and duration of benefit. Immunotherapy has emerged in recent years as a new therapeutic approach for BTC. However, the benefit with currently available treatments seems more modest than in other tumor locations and the predictive markers of response to these treatments remain to be identified. This review article summarizes the rationale, current data, and prospects and challenges for the development of immunotherapy in BTC.