The biggest challenge of the 21st century is to provide sufficient food, water, and energy to allow everyone on the planet to live decent lives, in the face of rising population, the threat of climate change, and (sooner or later) declining fossil fuels. In this talk Professor Sir Chris Llewellyn Smith (Director of Energy Research, Oxford University and President of SESAME Council) reviews the nature of the challenge and the portfolio of measures that must be adopted if it is to be met. These include lowering energy demand, through better planning and design and changes of behaviour, using energy more efficiently, the deployment of carbon capture and storage (if feasible and safe), and expansion of the use of renewable energy sources to the maximum extent reasonably possible. Major expansion of nuclear energy will be necessary. In the second half of the century thorium and/or fast breeder reactors, and/or large scale solar power, and/or fusion will be essential: all must be developed as a matter of urgency. The technical challenge is enormous, but the political and economical challenges are even greater.