The CASMO code is an assembly spectrum code that uses multigroup integral transport theory with four thermal energy groups for the two-dimensional calculation. The latest version, CASMO-3, contains several new features, specifically a new data library, multibundle capability, gamma detector calculation, flux discontinuity factor edit, and an option for generating transport theory corrected baffle/reflector data. Extensive benchmarking, including comparisons against cold- and high-temperature critical experiments, Monte Carlo calculations, measured isotopics, and power reactor core follow, has verified that accurate results are obtained. The new multibundle capability has been used for studies of interassembly spectrum coupling and for validation of core physics methods.