Sage is a free and open source, powerful and flexible computer algebra system. It provides integrated access to sophisticated mathematical software such as Pari, GAP and Maxima using a Python-based framework. The Sage project aims to reproduce the functionality (though not necessarily the syntax) of popular commercial CASs like MATLAB and Mathematica. This ideal is a moving target since MATLAB and Mathematica are under active development. But in any case Sage is already a mature project with substantial functionality. Sage has multiple interfaces, but this tech companion will focus almost exclusively on the standard notebook interface. You can download and install Sage at no cost (see the Sage Download page) or create an account, construct, save and run Sage Projects online at https://cloud.sagemath.com. You can even run Sage commands through some editable Sage Cells which appear in this TechCompanion.