At the end of inflation, dynamical instability can rapidly deposit the energy of homogeneous cold inflaton into excitations of other fields. This process, known as preheating, is rather violent, inhomogeneous and non-linear, and has to be studied numerically. DEFROST simulates preheating of the Universe after the end of the inflation. It is small, easy to modify, very fast, and fully instrumented for 3D visualizations. An MPI extension for this code, MPI-DEFROST (ascl:1106.022), is available.