Nearly five decades of research have established myosin as the main motor responsible for cytokinesis in organisms on the branch of the phylogenetic tree that includes amoebas, fungi and animals. This research has grown to be more mechanistic over the past decade, so we now have computer simulations of physically reasonable models that explain how myosins contribute to the assembly and constriction of contractile rings that pinch dividing cells into two daughter cells. Isoforms of myosin-II, from the same family as muscle myosins, are the main myosins for cytokinesis, but other myosins contribute to cytokinesis in fission yeast. Progress has been made on how animal cells use Rho-GTPases to control the accumulation and activity of myosin-II at the site of cleavage, but the regulatory mechanisms are less clear in other systems.