Due to their high energy density and zero emissions, hydrogen fuel cell UAVs have become a vital means of overcoming the endurance limitations of traditional lithium-battery-powered UAVs in the context of a growing low-altitude economy and the development of green aviation. This paper provides a comprehensive review and systematic categorization of the development and key technologies of hydrogen-fuel cell UAVs, along with a reasonable outlook on their future evolution, to serve as a valuable reference for related research. In particular, the paper starts with a summary of the scientific advancements and developmental history of hydrogen-powered aircraft, the inherent benefits of hydrogen fuel cell powertrain systems, and the state of the art in hydrogen fuel cell unmanned aerial vehicles. Subsequently, a systematic analysis is conducted on the core technologies involved in hydrogen fuel cell UAVs, including hydrogen fuel cell technology, multi-source hybrid energy systems, airborne hydrogen storage technology, energy management strategies, and thermal management strategies. Lastly, a summary of the study’s significant findings is provided, and future research directions are indicated by matching the state of these important technologies' research with the real-world difficulties they face.