The spin glass is a magnetic arrangement found in systems with randomly competing exchange interactions. It is characterized by local moments frozen in direction but with no long‐range order. A review if presented of recent theoretical formulations of the spin glass as a new magnetic phase. An integral part of the development is the recognition that a new type of order parameter is necessary for the spin glass and plays a non‐negligible role even for more conventional phases in disordered systems. Results are also presented of computer simulations which help to illustrate the physics and complement the experiments.