Aviation propulsion development continues to rely upon fossil fuels for the vast majority of commercial and military applications. Until these fuels are depleted or abandoned, burning them will continue to jeopardize air quality and provoke increased regulation. With those challenges in mind, research and development of more efficient and electric propulsion systems will expand. Fuel-cell technology is but one example that addresses such emission and resource challenges, and others, including negligible acoustic emissions and the potential to leverage current infrastructure models. For now, these technologies are consigned to smaller aircraft applications, but are expected to mature toward use in larger aircraft. Additionally, measures such as electric/conventional hybrid configurations will ultimately increase efficiencies and knowledge of electric systems while minimizing industrial costs. Requirements for greater flight time, stealth characteristics, and thrust-to-power ratios adds urgency to the development of efficient propulsion methods for applications such as UAVs, which looks to technologies such as asymmetrical capacitors to enhance electric propulsion efficiency. This book will take the reader through various technologies that will enable a more-electric aircraft future, as well as design methods and certification requirements of more-electric engines.