The paper gives an overview of the innovative field of hybrid energy storage systems (HESS). An HESS is characterized by a beneficial coupling of two or more energy storage technologies with supplementary operating characteristics (such as energy and power density, self-discharge rate, efficiency, life-time, etc.). The paper discusses typical HESS-applications, energy storage coupling architectures and basic energy management concepts including a hierarchical control- and optimization-based energy management. Four HESS-configurations, suitable for the application in decentralized PV-systems: (a) power-to-heat/battery, (b) power-to-heat/battery/hydrogen, (c) supercap/battery and (d) battery/battery, are presented along with a principle approach for the power flow decomposition based on peak shaving and double low-pass filtering. A modular experimental test-bed for hybrid energy storage systems is described in its components, structure and functionality.