Spotlight on Protein Folding: Proteins (green), chains of amino acids, are the nano-machines of life; some, for example, transport oxygen (red). Remarkably, based on their amino acid sequences, many proteins can spontaneously fold into unique 3D conformations necessary for function. Despite its complexity, the folding process is subject to simple general limits. Picture: M. M. Lin and A. H. Zewail, pp. 379–391, in this issue.