Battery energy storage systems (BESS) are increasingly installed in distribution grids in conjunction with other distributed energy resources (DER). These resources are solar photovoltaic (PV) systems and other distributed generation, including generation from renewable energy resources, such as wind and small hydro. One of the roles of BESS, in support of the wide deployment of renewable energy resources, including solar PV, is to meet the requirement for energy balancing to mitigate resource variability and intermittency and make the combination of BESS and renewable DER dispatchable. This balancing feature is applicable to smaller, rooftop PV installations (behind the meter) connected to the low-voltage (LV) grid and to larger PV or wind farms connected to medium-voltage (MV) distribution feeders. In the latter case, the combined installation feeds energy directly into the distribution system. BESS are key enablers for the implementation of active distribution system functions and for providing a range of grid services at the distribution level. Grid services include voltage support, feeder power flow control, load and hosting capacity management, local energy and capacity provision, and reliability and resilience services in the event of contingencies and exceptional atmospheric events. In this guide, it is assumed that these services are provided to the distribution system. Moreover, they can however be provided to support the operation of the transmission grid. BESS can be embedded in distribution systems in smaller units deployed in large numbers at low-voltage levels in conjunction with solar PV systems. In cases of wide deployment of a large number of distributed BESS and solar PV systems, these need to be aggregated and controlled using DER management systems (DERMS) to fully exploit their potential to provide grid support and services. This chapter addresses issues associated with BESS deployment, configuration, design, and management in the context of high penetration of renewable energy resources (RES) in distribution systems, including solar PV systems. It includes information and discussions of BESS design and deployment, interconnection and integration requirements, and aggregation of BESS with existing DER such as solar PV systems. It also addresses economic and business case considerations. Environmental impact, life expectancy, and life cycle considerations are discussed in relation to the different types of applications and BESS designs.