Reduction in green house gas emissions, increase in oil prices and dependency on foreign oil are major incentives to the development and deployment of Plug-in hybrid electric vehicles. The plug-in hybrid electric vehicle fleet is expected to increase the base electric load and add constraints on the reliable operation of a power system. However, equipped with bidirectional battery chargers, plug-in hybrid electric vehicles offer the power grid a flexible load, known as vehicle to grid mode of operation, capable of contributing to power system operations by supplying ancillary services. This paper gives an overview of the state of the art in battery technologies and charger requirements. The advantages and requirement for operating in vehicle to grid mode is presented. Alleviation of wind generation and improvement of short-term voltage stability are some of the potential benefits of the vehicle to grid mode.