{Regression testing \LaTeX\ packages with Lua} {Frank Mittelbach} {For many years, \LaTeXe\ has used a custom Perl script to perform regression testing on a large number of test files to ensure that any changes to \LaTeX\ do not break anything. This tests have also been useful in highlighting changes in \TeX\ Live. One of the first steps in the modern development of the \LaTeX3 code was to produce a similar system to ensure that the development process was as smooth as possible. This build system, which also handed documentation typesetting and \CTAN\ archiving, was written with a Makefile system, and eventually a Windows batch script was written as well. Nowadays, all \TeX\ distributions ship with Lua\TeX, which includes a standalone Lua interpreter; for the first time, \TeX\ users can write platform-independent scripts that run without needing to install any additional frameworks. (E.g., Perl under Windows.) Joseph Wright, accordingly, has rewritten the \LaTeX3 build scripts in Lua (uploaded to \CTAN\ in June 2014) to avoid maintaining two separate systems. As an added bonus, this new system, named \code{l3build}, is flexible and extensible enough to be used for any \TeX\ package, and we hope the community will now take advantage of having an easy-to-use regression test suite available. In this presentation, we will discuss how we use the \code{l3build} system, how we hope others will use it, and why regression testing is so important.}