Renaissance manuscript sources continue to tantalize with their rich repertories and mysterious origins. This chapter presents some of the ideas spawned by a re-evaluation of the Munich partbooks in light of new knowledge. The Munich partbooks are a substantial collection of 145 pieces, mostly German secular compositions, but also sacred pieces as well as French, Flemish and Italian works whose identity is often disguised by a lack of the original text. The image of Hieronymus Welser as a good-looking ne’er-do-well, interested in a young man’s pursuits of music, art, gambling and acquiring golden objects, takes on added significance in light of one of the great mysteries surrounding the Munich partbooks. Birkendorf has suggested that professional musicians could have used the Munich partbooks as raw material for new versions of pieces in the tradition of Fors seulement or Cela sans plus arrangements.