[en] Monte Carlo techniques are much used in neutron scattering studies both in the design of neutron scattering instruments and in the analysis of the data. Such codes require a common core of subroutines concerned with geometric and sampling problems and a collection of such routines is presented in this report. Using these routines it is possible to construct reasonable short MAIN programs (approximately 100 lines) capable of simulating simple systems such as a scintillator detector or neutron chopper. These subroutines have been developed on a DEC PDP 11/40 computer and include the non-standard FORTRAN function RAN(I,J) which provides random variable in the range [0,1]. When used on other computers a substitute routine must be provided. (U.K.)