The “batteries, hydrogen or renewable fuels?” question applies to aviation as well as to heavy-duty transport, though many more discoveries and inventions are needed for any of the three to power jet airliners. New battery technologies would need to greatly reduce the ratio of weight to energy from current lithium-ion architectures. Beyond the general cost issues of green hydrogen, affordable systems for safely storing hydrogen are paramount for sustainable aviation. Renewable fuels here face the same hurdle they do elsewhere: cost. Stanford researchers are working in all of these areas, though they are research topics in early development. In addition, some continue to work on making traditionally fueled airplanes more energy efficient, an area of great advances over the past several decades.