The past 15 years have seen a major increase in our understanding of the structure and dynamics of Bombyx mori silk fibroin, largely as a result of NMR studies. We now have a reasonably good idea of the structure before spinning (Silk I) and a good model for the crystalline regions after spinning (Silk II), though there are still some big outstanding questions. The details of the structures of Silk I and Silk II are the starting point for discussion of production of man-made silk fibroin, the origin of the strong silk fiber, and the mechanism of fiber formation in vivo.