Many materials possess an ordered array of magnetic moments at room temperature and above, due to interactions between electrons on neighbouring sites. In a ferromagnet, the moments align in the same direction, so that the material has a net magnetisation. There are several metals, in particular iron and nickel, whose ferromagnetism is due to interactions between delocalised, so-called itinerant conduction electrons. This chapter reviews general model of magnetic ordering, before turning to consider some of the features which enable the diversity of properties. When competing effects are present, the exchange interactions favour parallel spins and ferromagnetism in some cases, while favouring anti-parallel spins and antiferromagnetism in others. Despite the existence of spontaneous magnetisation below the Curie temperature, it is well known that ferromagnets can apparently lose their magnetisation. The unique attraction of permanent magnets is that they provide magnetic flux with no continuing expenditure of energy.