M-Unit, a tool that permits a series of tests to be written to address specific tags or entry points within an M project and act to verify that the return results are as expected for that code. If run routinely any time that the project is modified, the tests will act to indicate whether the intended function has been modified inadvertently, or whether the modification has had unexpected effects on other functionality within the project. The set of unit tests for a project should run rapidly (usually within a matter of seconds) and with minimal disruption for developers. Another function of unit tests is that they indicate what the intended software was written to do. This can be especially useful when new developers start working with the software or a programmer returns to a project after a prolonged period. Ensuring that well-designed unit tests are created for each project, therefore, assists development, enhances maintainability and improves end-user confidence in the deployed software.

The concept of Unit Testing was already in place before Kent Beck created a tool that he used in the language Smalltalk, and then was turned into the tool Junit for Java by Kent Beck and Erich Gamma. This tool for running specific tests on facets of a software project was subsequently referred to as xUnit, since NUnit was developed for .NET developers, DUnit for Delphi developers, etc. M-Unit is the equivalent tool for M developers to use and was originally created by Joel Ivey in 2003.