In this paper we discuss the principles of operation of polymer electroluminescent devices, and identify the factors which limit device efficiency. We identify how efficiencies can be improved by careful control of the polymer system, and by the use of multilayer structures to confine holes within the device. Using these techniques we can achieve efficiencies of better than 1% photons per electron in devices based on poly(p-phenylenevinylene). We also describe the use of induced absorption techniques to identify the excited states present within an electroluminescent device and to estimate their concentrations.