Relaxation is the result of turbulence in a plasma that behaves essentially as an ideal conducting fluid, but has a small resistivity and viscosity. These small effects are locally enhanced by the turbulence and lead to reconnection of magnetic field lines. This destroys an infinity of topological constraints, leaving only the total magnetic helicity as a valid invariant. The plasma therefore rapidly reaches a specific state of minimum energy. This minimum energy ‘‘relaxed state’’ can be calculated from first principles and has many striking features. These depend on the topology of the system. They include spontaneous field reversal, symmetry-breaking and current limitation in toroidal pinches, and flux generation and flux amplification in Spheromaks. In addition the relaxed states can be controlled and maintained by injection of helicity from an external circuit. These features, and the profiles of the relaxed states themselves, have been verified in many laboratory experiments.