Scattering is fundamental to almost everything we know about the world, such as why the sky is blue. Tunnelling is entirely quantum-mechanical and gives rise to such phenomena as nuclear fusion in stars. Examples and applications of both these fascinating concepts are investigated in this unit. 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.