The demands of exponentially growing Internet traffic, coupled with the advent of Dense Wavelength Division Multiplexing (DWDM) fiber optic systems to meet those demands, have triggered a revolution in the telecommunications industry. In the three short years of deployment, DWDM performance has accelerated dramatically. Channel counts have grown from 4 to 80, with 170 announced, and channel spacings have shrunk from 400 GHz to 50 GHz. Practical systems that put 1 TeraBit/sec. of information on a fiber are now on the horizon. This dramatic increase has been built upon, and has driven, improvements in fiber optic component technology, which has in turn driven improvements in photonic materials. The next generation of systems for the “all optical network” will require higher performance components coupled with dramatically lower costs. One approach to achieve significantly lower costs per function is to employ Planar Lightwave Circuits (PLC) to integrate multiple optical functions on a single substrate leading to a single package. In this way multiple components can be fabricated and interconnected at once, significantly reducing both the manufacturing and the packaging/assembly costs. The manufacture of PLCs, however, places demanding requirements on materials, design and fabrication processes. Parameters such as index of refraction, absorption and birefringence must be tightly controlled. PLCs have been made using inorganic crystals, such as Lithium Niobate, oxide glasses or polymers on silicon substrates and semi-conductor materials, such as Indium Phosphide (InP). All except InP are commercially available. In this paper we give an overview of the applications of PLCs in DWDM fiber optic transmission systems and discuss how material's requirements flow down from end-use requirements. The specific example of the use of polymer based thermo-optic switches for reconfigurable Optical Add/Drop Multiplexer (OADM) applications is discussed.