Poly (3-hydroxybutyrate) (PHB) is the most common bio-sourced linear polyester produced by bacteria, but suffers from extreme brittleness due to high crystallinity. PHB develops large spherulites as a consequence of a low nucleation density, and the picture shows PHB spherulites growing from the melt as obtained by polarized optical microscopy. As presented by Eric Dargent et al. on page 2586, the crystallization mechanisms are modified by adding a nucleating agent that makes the spherulites grow smaller and more homogeneous in size. Unfortunately, this kind of image is not possible when a nucleating agent is added, and models exist to obtain better insight into the crystallization mechanisms of nucleated PHB.