This chapter provides an overview of the causes of the different types of magnetic behaviour, and considers some of their consequences. It considers the behaviour of paramagnetic and diamagnetic atoms and solids, where the response of each atom or ion can generally be viewed as independent of all the others. Most solids are paramagnetic or diamagnetic, with a very weak response to an applied magnetic field. The chapter describes the more interesting case of ferromagnets, ferrimagnets, and antiferromagnets. The additional interactions occurring between electrons on neighbouring ions enable the electrons to respond in a cooperative manner to an applied field, allowing much stronger magnetic effects. The chapter shows how an individual electron has an associated magnetic moment. The Langevin theory provides a good description of the behaviour of paramagnetic materials. The chapter compares the experimentally determined variation of the magnetisation for several paramagnetic salts with the theoretically predicted variation, using the Brillouin function.