"Because they know what they do" should be the contribution of cancer registration to prevention. The public should be informed about the successes and failures of prevention. In addition, each doctor and each hospital should know the long-term results for its patients despite the complex interdisciplinary health care provision. At the same time, the regional results should be available and contrasted with clinical studies and international standards. An important criterion is also the quality of life of the patients, whose cooperation is more than overdue. According to the possibilities of prevention, six important levels can be differentiated. On each level the outcome should be evaluated on the basis of slightly differing criteria. Primary prevention has the largest incidence-, and thus, mortality-reduction potential, essentially caused by the giving up of smoking. However, the primary prevention of the tumor depends upon what kind of tumor it is, as primary prevention is possible to different extents. Often, secondary prevention is the earliest intervention possible, that is, the early detection of cancer in prognostically favorable stages. However, early detection only offers a chance of cure, albeit a smaller one, for some kinds of tumor. The third prevention level comprises primary care according to the state-of-the-art standards, followed by posttreatment care (disease-free phase) even when the disease is running a fatal course (palliative phase). In the terminal phase, adequate tumor-pain therapy and symptom-oriented measures stand for the sixth prevention level. Even the quality of life of the dying can be optimized. At the beginning of treatment at the latest, the quality of life should also be added to the outcome criteria. In this paper, the individual levels of prevention and possible evaluation criteria for successful prevention, which a modern cancer registration should submit, are discussed and examples given. If the evaluation of the quality of treatment and its significance for health care provision becomes the central tasks of the cancer registers, they will need to be transformed into service centers for hospitals, doctors, and patients.