F1-ATPases are large multimeric proteins that can be isolated from the membrane bound system that catalyzes the phosphorylation of ADP by inorganic phosphate in bacteria, plants, and mitochondria. They can be visualized in electron micrographs of the inner mitochondrial membranes where they appear as large protruding spheres 90 A in diameter. The purified F1-ATPases have a molecular weight of 320,000 to 400,000 daltons and are composed of five non-identical subunits (alpha, beta, gamma, delta and epsilon). The stoichiometry of these subunits in the complex is still unknown but compositions of the type alpha3beta3gamma delta epsilon and alpha2beta2gamma2delta2epsilon2 were found to be consistent with some of the available experimental data. This review discusses the recent data and the experimental approaches utilized for the structural characterization of F1-ATPases.