McStas[1][2] is free and open-source (GNU General Public License) software simulator for neutron scattering experiments.

A special compiler translates a domain-specific language describing the neutron instrument geometry and component definitions (written in C) to a stand-alone C code.

The basics of McStas was written in 1997 at Risø for simulation of their neutron experiments, that were based at the DR3 reactor that was shut down in year 2000.

The Copenhagen-based Data Management and Software Centre of the European Spallation Source is also expected to become a partner since many of the future instruments are being simulated using McStas.

McXtrace, an equivalent simulation package using X-rays instead of neutrons, started being developed in 2009 and it is now freely available.