Cancer of the gallbladder, a rare entity with a poor prognosis, is often discovered incidentally during or after cholecystectomy. It tends to disseminate early via lymphatic, peritoneal, endobiliary, and hematogenous pathways. Diagnosis is made intra-operatively in only a quarter of cases, by examination of the opened cholecystectomy specimen in the operating room by the surgeon; this procedure should be routine. For incidentally-discovered cancers, survival was 28% at five years. Prognostic factors include age, TNM stage, gallbladder perforation during cholecystectomy and less-than-optimal resection at re-operation. Whether the laparoscopic route for the initial cholecystectomy has an impact on survival remains a subject of debate. R0 surgery is the only potentially curative treatment: simple cholecystectomy with clear margins is adequate resection for stage T1a tumors; extended cholecystectomy with lymphadenectomy and possibly resection of the bile duct is required for more advanced stages. After curative resection, neo-adjuvant or adjuvant chemotherapy and radiotherapy have not, so far, proven effective. Improvement of surgical practices (systematic review of cholecystectomy specimens in the OR, prevention of gallbladder perforation with bile spillage during surgery, early re-intervention for optimal resection) could improve the prognosis of these cancers.