Access to safe, clean and sustainable energy supplies is one of the greatest challenges facing humanity during the twenty-first century. This unit will survey the world’s present energy systems and their sustainability problems, together with some of the possible solutions to those problems and how these might emerge in practice. This study unit is just one of many that can be found on LearningSpace, part of OpenLearn, a collection of open educational resources from The Open University. Published in ePub 2.0.1 format, some feature such as audio, video and linked PDF are not supported by all ePub readers.