The stabilization of Blue Phases has recently attracted much interest for the production of fast displays based on the Kerr effect. One method to stabilize Blue Phases, and frustrated liquid crystal phases in general, is by dispersing monomers and subsequent photo-polymerization. In this chapter we discuss the effects of polymer networks on the width of the generally very narrow Blue Phases, as well as Twist Grain Boundary (TGB) phases. Within the framework of polymer modified liquid crystals, the method of dispersing polymers in frustrated phases is also discussed.