The first measurements with arrays of fissile metal were performed at the Los Alamos Scientific Laboratory in 1947 and the first with fissile solutions were performed at the Oak Ridge Critical Experiments Facility in 1949. Since then, there have been many other significant experiments at several U.S. laboratories including, in addition, Rocky Flats, Battelle-Pacific Northwest Laboratory, and the Lawrence Livermore Laboratory. Array tests were the primary sources of data used in developing criticality criteria for fissile-process plants, and they provided the basis for several empirical storage models that are still in use. Some of the experimental data also serve to validate Monte Carlo neutron transport calculations now used extensively by the nuclear safety engineer in the design of storage and processing facilities. The authors feel that there should be additional experimental data for further validation of calculational methods relied upon for criticality safety evaluation. The deficient areas include low-235U-enriched uranium, 233U with and without thorium, and plutonium-uranium mixtures. Also, critical data are lacking for arrays with the concrete reflectors normally found in process-plant environments , and additional experiments on concrete-reflected arrays are needed.