It is widely recognised that fundamental change in energy systems is needed to address the economic (e.g. volatility of fossil fuel prices), social (e.g. fuel poverty), political (e.g. energy security) and environmental (e.g. climate change) problems associated with current systems of energy provision. While much of the energy policy literature (and policy-making) so far has focussed on the supply side, energy efficiency is one of the key options to make energy systems more sustainable and can provide a number of benefits, including increasing energy security, contributing to economic growth and job creation, reducing fuel poverty, leading to savings in public expenditure and reducing carbon emissions. As a consequence, interest in energy efficiency has grown significantly over the last few years, both in policy making as well as in academia. This chapter will reflect on energy efficiency policy and its potential contribution to stimulating transitions towards more sustainable energy systems. It uses the case of the UK to illustrate issues of relevance for policy makers and analysts across the EU and the US.