Flagella are responsible for bacterial motility and chemotaxis. They are subcellular organdies that originate in the membrane of the cell and extend 15-20 µm from the cell surface. The bacterial flagellar system has been studied from a variety of points of view, including as a model for the regulation of organelle formation and morphogenesis, as a system that transduces chemical energy into motion, and as the basic effector involved in chemotaxis. To frame precise questions in each of these areas, it is necessary that a great deal be known about the nature of the components and the behavior of the system. Recent work has led to a clearer characterization of structure and function of bacterial flagella in Escherichia coli and in Salmonella. Our objective in this review is to summarize the status of current work, as well as to identify some of the new questions this work raises. We are selective, particularly in the area of chemotaxis since comprehensive reviews of recent (4, 25, 85, 101) well as earlier work (24) in this area have appeared. Flagellar assembly and the regulation of flagella formation have received somewhat less attention. The most recent extensive reviews were by Iino (63) and Doetsch & Hageage (47). ; © 1977 by Annual Reviews Inc. This research was written during the tenure of a grant to the authors from the National Science Foundation.