The returns of a tax called the common penny (gemeiner Pfennig) survive in considerable number, and when they are fully exploited they will help to make pre-Reformation Germany much more intimately known than ever before. Registers of this levy are likely to turn up in archives where they might not be expected. It is important, as a result, to know how to recognize and use these materials, most of which still remain to be identified. I will briefly review the common penny and its collection, and then I will demonstrate its potential as a resource for the study of social and demographic history.