Biliopancreatic maljunction, also known as pancreaticobiliary maljunction or an anomalous junction between the pancreatic duct and the bile duct (APBDJ) is a congenital anomaly that has been defined as a union of the pancreatic and biliary ducts that is located outside of the duodenal wall. Because the function of the sphincter muscle of the duodenal papilla does not extend the full length of the common channel, this results in the regurgitation of the pancreatic juice and bile. In most cases, since the pressure within the pancreatic duct is higher than that of the biliary tract, the pancreatic juice regurgitates continuously into the biliary tract and the biliary mucosa is continually susceptible to damage as a result of the continued presence of infectious bile and activated pancreatic enzymes. This eventually causes cancer and other pathological changes to occur in the biliary mucosa. Since APBDJ can cause various diseases and conditions in the pancreas as well as the liver, further research is needed in this field. With developments in endoscopic retrograde cholangiopancreatography (ERCP), the morphology of APBDJ has been clarified and it was soon discovered that some cases of APBDJ are not accompanied by dilatation of the extrahepatic bile duct. APBDJ is very closely related not only to congenital biliary dilatation, but also to the occurrence of cancer in the biliary tract, and so many researchers, including those whose research is focused on the biliary tract and pancreas of adults, have participated in conferences on this subject. Much data have been collected from various groups and fundamental research in a wide range of areas has been conducted.