The UK economy needs to be decarbonised by a factor of 10 in the first half of this century. This is a monumental change requiring significant acceleration of the underlying processes by which advanced economies tend to decarbonise. It is a major systems challenge, covering measures for much improved energy efficiency and demand management; many sources of primary energy; energy storage; the transmission and distribution of energy in a number of different forms; low carbon processes, devices and buildings; and measures for mitigating environmental impacts. The energy system needs to be optimised to deliver low carbon goals at affordable least cost. Other factors such as energy security and economic development are also relevant to determining systems solutions. This is not only a technological and engineering challenge. Institutional frameworks and the best mechanisms for translating high-level systems goals into detailed commercial incentives are crucially important.