Correlation is the tendency of two or more systems that independently exhibit simple behavior to show complex and novel behavior together because of their interaction. In a wide sense, correlation is responsible for most of the complex behavior of matter observed at all scales. Examples include the forces that confine quarks inside a particle such as the proton, the interactions responsible for superconductivity, the energy landscapes that govern chemical reactions of molecules, and the principles that govern the shapes and functions of giant protein molecules in living systems.