An improved therapy planning--for conservative as well as for surgical treatment--depends on an exact diagnosis of the ankylosis. Above all, it is decisive to know whether the ankylosis has intraarticular or extraarticular causes or if there are obstacles in form of osseous parts or soft tissues. We dispose of the following diagnostic means: the clinical manifestations and the X-ray radiography. In most of all cases, these criteria are sufficient to make an exact diagnosis. Complementary examination methods such as arthrography, arthroscopy and, last but not least, thermography are only needed in exceptional cases.