The lock-and-key model first postulated by the famous chemist Emil Fischer proposes that enzymes and their substrates must fit together like locks and keys in order to fulfill their physiological functions. Even though it has long become clear that biological systems are nowhere near as rigid as their metallic namesakes, the principle of molecules interacting together three-dimensionally is the basis for almost all biological processes. Of course this is not only true for enzymes, but for all types of protein interactions. Only a protein that is correctly folded and structurally intact can do its job properly. For researchers working with proteins, it is therefore fundamentally important to be able to analyze both protein structure and function.