This book discusses innovations in the field of hybrid energy storage systems (HESS) and covers the durability, practicality, cost-effectiveness, and utility of a HESS.  It demonstrates how the coupling of two or more energy storage technologies can interact with and support renewable energy power systems.  Different structures of stand-alone renewable energy power systems with hybrid energy storage systems such as passive, semi-active, and active hybrid energy storage systems are examined.  A detailed review of the state-of-the-art control strategies, such as classical control strategies and intelligent control strategies for renewable energy power systems with hybrid energy storage systems are highlighted. The future trends for combination and control of the two systems are also discussed.