UNIT: A freely available system identification toolbox. This paper presents a Matlab-based software package for the estimation of dynamic systems. It has been developed primarily as a platform to support the objective evaluation of novel approaches relative to existing methods within a common software framework. This is designed to streamline comparisons. The work here provides an explanation of the toolbox’s design and use together with an overview of the underlying supported models and algorithms. An application study involving a lightly damped resonant system and a simulation example involving two non-linear systems are also presented in order to illustrate the use and capabilities of the toolbox.