Over fifty fast critical assemblies have been studied in the Zero Power Reactor - III (ZPR-III) of Argonne National Laboratory since it started operation in 1955. All of these assemblies were fueled with U235 and reflected with depleted uranium; the core volumes ranged from 2 to 660 liters and the critical masses ranged from 26 to 580 kg of U235. The experimental characteristics of a representative group of 23 of these assemblies in which oxide, carbide and metallic fuels were simulated have been compared with calculated values. The parameters studied were critical size, central fission ratios, prompt-neutron lifetimes and the reactivity effects of substitution of various materials at the reactor center.