Battery energy storage can be used for a variety of applications, including frequency regulation, demand response, transmission and distribution infrastructure deferral, integration of renewable energy, and microgrids. Different battery technologies can enable different applications that can provide various benefits to the utility services, Independent System Operator (ISO) services, Regional Transmission Organization (RTO) services, and consumer services. This report by NRECA, in conjunction with CFC, CoBank, and NRTC, reviews two principal technologies that are the leading battery energy storage deployment - lithium-ion and flow batteries - and provides co-op case studies on battery energy storage application.