We investigate the influence of particle number nonconserving processes on the thermal history and the dynamics of the universe. There occurs a backreaction both on the temperature and on the scale factor if particles of one species decay into particles of another species, or if the overall particle number changes. This backreaction is determined by the ratios of the relevant decay and production rates to the Hubble expansion rate. It results in reheating phenomena and modifications of the standard Friedmann-Lemaitre-Robertson-Walker (FLRW) expansion laws. A high production rate at the Planck scale tends to avoid the cosmological singularity. © 1994 Plenum Publishing Corporation.