MCNP is a very general Monte Carlo neutron-photon transport code with approximately 250 person years of Group X-6 code development invested. It is highly portable, user-oriented, and a true production code as it is used about 60 Cray hours per month by about 150 Los Alamos users. It has as its data base the best cross-section evaluations available. MCNP contains state-of-the-art traditional and adaptive Monte Carlo techniques to be applied to the solution of an ever-increasing number of problems. Excellent user-oriented documentation is available for all facets of the MCNP code. Many useful and important variants of MCNP exist for special applications.