Multistage testing, or MST, was developed as an alternative to computerized adaptive testing (CAT) for applications in which it is preferable to administer a test at the level of item sets (i.e., modules). As with CAT, the simulation technique in MST plays a critical role in the development and maintenance of tests. "MSTGen," a new MST simulation software tool, was developed to serve various purposes ranging from fundamental MST research to technical MST program evaluations. "MSTGen" runs on a Microsoft Windows-based operating system with .NET framework 2.0 or higher. This article discusses the advantages of "MTSGen" and what it supports.