Only 10 years ago the idea of reliably operating a multitude of different radio sensors solely with ambient energy seemed unreal. These days, however, if one enters the term "energy harvesting" into an internet search engine, one gains about half a million hits. No doubt, the subject has become more than simply another topic of conversation. Surprisingly, however, only very few products exist on the market that hold potential for high unit numbers in mass applications. The article outlines significant technical challenges and their solutions on the way to producing energy autarkic radio sensors in large scale, using select products of EnOcean as examples. In so doing, it becomes clear that innovative technical concepts oftentimes force one to "reinvent" all parts of a system, thus further accelerating technological progress.