AREPO is a massively parallel code for gravitational n-body systems and hydrodynamics, both on Newtonian as well as cosmological background. It is a flexible code that can be applied to a variety of different types of simulations, offering a number of sophisticated simulation algorithms. An description of the numerical algorithms employed by the code is given in the original code papers (Springel 2010, MNRAS, 401, 791; Pakmor et al. 2011, MNRAS, 418, 1392; Pakmor and Springel 2013, MNRAS, 432, 176; Pakmor et al. 2016, MNRAS,455,1134) and the release paper of this version (Weinberger et al. 2019). A user guide can be found under `/documentation`, which also includes a 'getting started' section, which is recommended for new users. An html version of the user guide can be created usingsphinx (https://www.sphinx-doc.org) by typing cd ./documentation/ make htmland displayed by opening `./documentation/build/html/index.html`.A full version of the user guide is also available on the Arepo homepage.