The NEXUS Class Library (NCL) is a collection of C++ classes designed to simplify interpreting data files written in the NEXUS format used by many computer programs for phylogenetic analyses. The NEXUS format allows different programs to share the same data files, even though none of the programs can interpret all of the data stored therein. Because users are not required to reformat the data file for each program, use of the NEXUS format prevents cut-and-paste errors as well as the proliferation of copies of the original data file. The purpose of making the NCL available is to encourage the use of the NEXUS format by making it relatively easy for programmers to add the ability to interpret NEXUS files in newly developed software.