Since the discovery of electroluminescence in conjugated polymers it has been recognised that charge transport is a key ingredient for the performance of polymer light-emitting diodes (PLEDs). It is demonstrated that at low voltages the quantum efficiency of a PLED is limited by the unbalanced electron and hole transport in the polymer. The response time of a PLED, which is relevant for its use in pixilated displays, is governed by the transit time of holes towards the cathode. The hole transport in various derivatives of poly(-p-phenylene vinylene) (PPV) is characterised by a combination of space-charge effects and a field- and temperature dependent mobility. The specific form of the mobility provides information on the relation between chemical composition of the polymer and microscopic charge transport. From this relation the power efficiency of PPV-based LEDs has been optimised.