This chapter explores the principles behind magnetic materials. The great advantage of magnetic materials is that they can store so much information in such a small space. The magnetic properties of a material can be understood in microscopic terms when we consider the special properties of moving electrons. It is for this reason that electrical and magnetic properties are related and some of the terms are parallel in their use. In contrast with diamagnets and ordinary paramagnets, a ferromagnet relies on more than the single-atom properties to derive its interaction with a magnetic field. In a ferromagnetic metal, the importance of many-atom effects in determining magnetism is directly related to the electronic band structure. Chemists have been designing new magnetic materials with molecular building blocks to have control over their properties. Magnetic fields can influence the energy states of atoms and ions, and this situation can be used to probe the structure and dynamics of materials.