Cryogenic fuels are fuels that requires storage at extremely low temperatures. Cryogenic fuels most often constitute liquified gases such as hydrogen. Quite often, liquid oxygen is mistakingly called "cryogenic fuel" as well, though it is actually an oxidizer and not a fuel.
Some rocket engines are cooled by circulating their cryogenic fuel around their nozzles before the fuel is pumped into the combustion chamber. This arrangement was first suggested by Eugen Sänger in the 1940s.