A concept from the field of quantum mechanics, spin is a property of particles that has no direct counterpart in everyday experience. Since the early 20th century, researchers have learned to manipulate it deliberately by means of precisely shaped electromagnetic pulses. This has resulted in two technologies that are widespread today: magnetic resonance imaging (MRI) is commonplace in hospitals, and nuclear magnetic resonance spectroscopy (NMR) is used to determine the structures of proteins and to develop new drugs. The mathematical framework of optimal control theory can be used to improve these technologies.