Battery electric energy storage systems (BESS) are increasingly connected to electric transmission and distribution networks to implement functions such as balancing renewable energy resources, load leveling and shifting and serving as back-up power for energy sources. In distribution networks, they also provide operational flexibility, allow deferral or elimination of the need for capital expenditures to upgrade networks, enable greater integration of renewables and generate service revenue. These benefits can be exploited in all types of electricity distribution grids, including grids connected to strong central power plants, grids connected to weaker grids and remote and autonomous grids. In the case of connections to weak central grids, BESS can play an important role in operations and increase the reliability and resilience of distributions grids. Among others, they allow balancing of local renewable energy resources and islanded operations and may work in parallel or replace conventional generation, often based on fossil fuels. Batteries have been employed for many years for back-up generation and are a proven technology. Their integration into the power systems as a support for additional generation provided by renewable energy resources and/or ancillary services is more recent. This chapter addresses the following topics: