This book introduces recent developments in green propellants and green propulsion systems by various worldwide research groups. Various space agencies across the globe have accelerated the development of non-toxic green propellants, and the European Union is planning to phase out hydrazine (liquid) and ammonium perchlorate (solid) as propellants in the future, as it was identified as a substance of high concern by the REACh regulation. Although the process is still ongoing, it has triggered the development of attractive new propellants and technologies for the market, fostering research for non-toxic, high-performance alternative technologies. Several green propellants and green propulsion systems have been developed, tested, and even commercialized, such as ADN- or HAN-based propulsion, hydrogen peroxide propulsion, and water propulsion technology, where water is electrolyzed in orbit to O2 and H2, then combusted for the generation of thrust. This book describes proposed solutions to replace hydrazine, its derivatives and ammonium perchlorate with green propellants, discusses recent progress in environmentally friendly propellant systems and covers recent advancements on materials, catalysts and oxidizers for green solid rocket propellants. It aims to provide necessary technical information for space mission and propulsion systems designers and researchers in this aerospace field.