Nek5000 is designed to simulate laminar, transitional, and turbulent incompressible or low Mach-number flows with heat transfer and species transport. It is also suitable for incompressible magnetohydrodynamics (MHD). Nek5000 is written in Fortran 77 and C. It uses MPI for message passing (but can be compiled without MPI for serial applications) and some LAPACK routines for eigenvalue computations (depending on the particular solver employed). Nek5000 output formats can be read by either postx (distributed with Nek5000) or the parallel visualization package VisIt developed by Hank Childs and colleagues at LLNL/LBNL. VisIt is mandatory for large problems (e.g., more than 100,000 spectral elements).