The concept of ”auf Kosten” (infringement on another person's rights which causes detrimental impact on such other person's interests) of ”Eingriff” (the type of infringement) within the theory of quantum meruit has been a difficult and controversial issue. The development of the theory started from ”Rechtswidrigkeit”, then to ”Zuweisungsgehalt”. Much discussion has occurred on this subject and has spurred the development of new theories. This article would like to introduce and analyze the pros and cons of traditional ”Rechtswidrigkeit” theory and ”Zuweisungsgehalt” theory. In addition, I would like to discuss new theory recently developed, and wish to determine the most appropriate ”Eingriff” theory for quantum meruit.