This chapter discusses the development of electroluminescent polymer systems. A typical design of a polymer light emitting devices (LED) is shown. Light emitting device and mechanism for light emission are discussed. The first highly structured electroluminescent polymer, poly(p-phenylene vinylene) (PPV), a green-yellow emitter, was prepared via a precursor route because its insolubility in poly reactions resulted in only oligomeric materials. An important soluble precursor route for PPV and related polymers involves the polymerization of 1,4-bis(chloromethyl or bromomethyl) arenes by treatment with potassium-t-butoxide in nonhydroxylic solvents like tetrahydrofuran. Illustrative examples of EL structures exploring the concept of conjugation confinement are shown. The synthesis of polymers with high electron affinity as the solution processable poly(cyanoterephthalydene)s which are derivatives of PPV with cyano groups attached to the vinylic carbons has provided the material necessary to complement the existing hole transport PPVs. Poly(p-phenylene) (PPP) is an interesting material for electrooptical applications. Polyfluorenes were introduced as a prospective emitting layer for polymer LEDs. Polythiophene and substituted polythiophenes can be prepared by chemical or electrochemical routes. Due to their strong electron-acceptor character, nitrogen-containing groups of various kinds have been incorporated to conjugated polymeric structures. The interest in silicon-based polymers resides in the delocalization of the σ electrons over a Si backbone providing electronically analogous properties to the π-conjugated polymers.