Bone Tumors - A Challenge for Cooperation E. GRUNDMANN Among the wide variety of human tumors, those of the skeletal system have an exceptional position in several respects. Above all, they are comparatively rare, and that is why reliable diagnostic criteria were compiled only recently, that is during the last three decades. It is only five years since the outlines of an international code of classi­ fication were traced. The code was applied and discussed critically and with varying results by several international working groups. Cer­ tain drawbacks are due to the broad and manifold spectrum of histolo­ gic manifestations in neoplastic bone. Even the best experts in dia­ gnostic histology would hardly be able to classify more than 85% of all bone tumors after the proposed code system. For quite a long time the osteoclastic giant cells, observed in almost any kind of bone tumor, have so much fascinated the histologist's eye that he was apt to pro­ claim the diagnosis "brown giant cell tumor" for almost any osteoclas­ tic tumor regardless of its benignity or malignancy. Nowadays the group of "giant cell tumors" though'restricted, still remains disputed in terms of malignancy and benignity. The problem will certainly not be settled in a definite way without extensive follow-up studies. In this context we may see actual progress in new regional. bone tumor registries being set up after the American example in many countries, among them the German Federal Republic.