Abstract. Effective theories have been very successful in all branches of theoretical physics. There are still a variety of phenomena where we do not have a clear understanding from the standard effective field theory point of view. One such phenomenon is dissipation and information loss in time evolution of a system. In the open quantum system literature little has been done for field theoretic systems. In this talk what we will do is first explain the tools available to deal with effective field theories of open systems. Then we will use these tools to study open version of \phi^3+\phi^4 theory, discuss few interesting aspects and study renormalisability of this theory.