This book presents an overview of the operation of Distributed Energy Resources (DER) and their role in enabling active distribution networks. It is based on the work of SC C6 carried out within its working groups in the past 5 years. Included are descriptions of DER assets, network systems, planning and operation, rural and off-grid systems and future prospects. Describing international developments and including several DER application examples, this book will provide a useful guide to researchers and practitioners interested in the subject.