New insights into the workings of the ultimate swimming machine.

Over the last few years, increasingly clever in vitro motility assays have allowed mechanical and kinetic measurements at the level of single molecular motors (see for example Block and Svoboda (1995), Finer et al. (1995), and Yanagida and Ishijima (1995) in a special issue of Biophysical Journal devoted to molecular motors). But to get a handle on complex phenomena such as cytokinesis or muscle contraction, we need to understand how molecular motors work in ensembles from a few to