This book reports on research and developments towards sustainable, climate-neutral, and resilient aviation. Gathering peer-reviewed contributions to the 2nd Vienna Aviation Days, held on July 7-8, 2025, in Vienna, it covers findings from European research projects funded under the Horizon Europe programme and the ClimAvTech Cluster. It presents numerical and multidisciplinary optimization models, advances in materials and additive manufacturing, engine design methods and agile project management strategies, fostering developments and industrial applications of hydrogen, electric and hybrid/electric aircraft propulsion systems. A special emphasis is given to short- and medium-range aircraft and propulsion systems, which are responsible for more than 50% of the global aviation emission. All in all, this book provides researchers from academia and industry with extensive and timely information on state-of-the-art developments in carbon-free aviation.