The results of the International Atomic Energy Agency cross-section processing code verification project are described. This project has been conducted over the last 7 yr in an attempt to improve the reliability of the results produced by cross-section processing computer codes. Initial comparisons of results received from 42 participants using 13 different cross-section processing codes demonstrated that, even though all of the codes started from exactly the same evaluated data, in no case did the multigroup cross sections calculated by any two codes agree for all materials and groups used in the comparison. This code verification project has led to positive results by improving cross-section processing codes as well as the conventions used to interpret ENDF/B evaluations. In several cases it has led to changes in ENDF/B evaluations. Although this project only addressed nuclear data processing codes, conclusions can be drawn from the results regarding the reliability of other types of computer codes.