McStas is free and open-source (GNU General Public License) software simulator for neutron scattering experiments. McStas is an abbreviation for M onte c arlo S imulation of t riple a xis s pectrometers, but the software can be used to simulate all types of neutron scattering instruments. The software is based on both Monte Carlo methods and ray tracing. 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.