The current implementation of self-gravity in Enzo uses a fast Fourier technique (Hockney & Eastwood 1988) to solve Poisson’s equation on the root grid on each timestep. The advantage of using this method is that it is fast, accurate, and naturally allows both periodic and isolated boundary conditions for the gravity, choices which are very common in astrophysics and cosmology (with isolated boundary conditions on the root grid being implemented with the James (1977) method). On subgrids, we interpolate the boundary conditions from the parent grid (either the root grid or some other subgrid). The Poisson equation is then solved on every timestep using a multigrid technique on one subgrid at a time. Aside from self-consistently calculating the gravitational potential arising from the baryon fields and particles in the simulation, there are also a number of options for specifying static gravitational fields (including, for example, gravitational acceleration from NFW halos, galactic disks, and point sources). Enzo parameters relating to gravity can be found in Gravity Parameters, and a brief description .