Interest in reactive extrusion has grown considerably during the last decade. The instabilities that may occur during polymerization in extruders can pose severe limitations on the process and on its scalability. They manifest themselves as severe fluctuations in throughput and product quality or as dual steady states, one at low conversion and one at high conversion. These instabilities may be of thermal, hydrodynamic, or chemical nature. In this paper the possible sources of instabilities are identified in a qualitative way and some possibilities to prevent them are given. Because some of the instabilities are dependent on the scale of the equipment, experiment design is important when a reactive extrusion process has to be developed on laboratory equipment and successively scaled up to larger machines.