The purpose of this review is to provide an up-to-date summary of E. coli proton-translocating F1F0ATPase. From work on this enzyme, new insights have been gained in the areas of bacterial physiology and energy metabolism, mechanism of enzyme action, mechanism of ion transport through membranes, structure of membrane proteins, mechanism of energy coupling, and regulation of complex enzyme expression and assembly. An important and pressing need is for more structural information. High-resolution structural analyses of F1F0 have not progressed far, and this is likely to present a road block unless overcome. One possibility is to crystallize or apply nuclear magnetic resonance spectroscopy to isolated subunits available in native form from E. coli F1F0. In this way, one might incrementally build a structure of the F1F0 complex. Static views, however, are unlikely to provide a complete picture of a dynamic enzyme such as this, in which long-range interactions between F0 and F1 and cooperative interactions between nucleotide-binding sites play such an important role in catalysis. Mutagenesis and reversion analysis are two powerful techniques, which, combined with direct enzymological measurements, can be exploited in the immediate future to study the intriguing dynamic aspects of F1F0 function. Many questions remain to challenge us. Regulation of enzyme activity in the cell is not understood. The role of the noncatalytic nucleotide sites is unknown. The assembly pathway and regulation of expression are not established. The mechanisms of H+ translocation and catalysis seem to be proving amenable to analysis, and further advances in these areas can be expected. Long-range conformational interaction between the H+ conduction machinery in F0 and the catalytic sites in F1 seems basic to energy coupling; a major future goal is to provide a realistic physical explanation to validate this concept.