Technology advancements enable and encourage higher system electrifications in various applications. More electrified applications need more capable and higher performing sources of energy in terms of power delivery, power regeneration, and energy capacity. For example, in electric, hybrid electric, and plug-in hybrid electric vehicle applications (EVs, HEVs, and PHEVs), the power and energy ratings of the vehicle energy storage system (ESS) have a direct impact on the vehicle performance. Many researchers investigated and studied various aspects of hybrid energy storage systems (HESS) wherein multiple ESSs are combined together to share system loads, increase ESS capabilities, and cycle life. Various configurations and their application specific topologies were also proposed by other researchers; the potential of HESS has been proven to be very promising. In this research, the goal is to present the theory of a HESS configuration that has not been discovered thus far. This HESS configuration is called a series-parallel reconfigurable HESS (SPR-HESS) since it is capable of recombining multiple storage systems into different series, parallel, or series-parallel configurations, via power electronic converters, to accommodate different operation modes and load requirements. Simulations, as well as experimental verifications, are presented in this thesis. / Thesis / Doctor of Philosophy (PhD)