Achieving supply demand balance in power systems requires controllable energy storage. The primary sources of controllable storage are the fuel stockpiles at generators (i.e. gas, coal, water, etc.). There is increasing interest in alternative forms of storage due to factors such as the continuing increase in intermittent energy generation sources, concerns with increasing network costs and demand side management. This paper reviews the need for and provision of the various forms of energy storage within power systems with particular emphasis on intermediate storage in which electrical energy is converted, in a controlled manner, to potential energy in some form and later recovered in the form of electrical energy. The potential applications, operation, and benefits of controllable intermediate storage are reviewed.