The second edition of the WHO Histological Classification of Tumors of the Gallbladder and Extrahepatic Bile Ducts is more comprehensive and detailed than the previous one. Advances in our understanding of dysplasia, carcinoma in situ, various lines of differentiation among the carcinomas, and the recognition of a variety of tumor-like lesions have resulted in more than three times as many entities in the current classification as in the previous one. The new edition should facilitate pathologic, epidemiologic, and therapeutic comparisons.