Arepo is a massively parallel gravity and magnetohydrodynamics code for astrophysics, designed for problems of large dynamic range. It employs a finite-volume approach to discretize the equations of hydrodynamics on a moving Voronoi mesh, and a tree-particle-mesh method for gravitational interactions. Arepo is originally optimized for cosmological simulations of structure formation, but has also been used in many other applications in astrophysics.