Nek5000 is a computational fluid dynamics code that simulates unsteady incompressible fluid flow with thermal and passive scalar transport. It can handle general two- and three-dimensional domains described by isoparametric quad or hex elements. In addition, it can be used to compute axisymmetric flows. Nek5000 won a Gordon Bell prize for its outstanding scalability on high-performance parallel computers. It currently has more than 160 users and is a part of the CESAR co-design effort.