How monomers of the cytoskeletal protein actin join to form the stable polymers crucial to muscle contraction and cellular motility has been a long-standing question. A state-of-the-art approach provides an answer. The actin protein is abundant in all eukaryotic cells (those characterized by a membrane-bound nucleus), and is particularly prevalent in muscle, where it comprises about 20% of the total mass. Actin comes in two forms: monomeric, globular G-actin; and polymeric, filamentous F-actin.