A combined gas turbine engine and hydrogen fuel cell system comprises a hydrogen fuelled gas turbine engine (203), a cryogenic liquid hydrogen fuel tank (104), a first fuel offtake (220) configured and arranged to divert a portion of hydrogen fuel from a main fuel conduit (217), a burner (222) configured and arranged to burn the portion of hydrogen fuel diverted from the main fuel conduit (217), a heat exchanger (224) configured and arranged to transfer heat from exhaust gasses produced by the burner (222) to hydrogen fuel in the main fuel conduit (217), a second fuel offtake (262) arranged to divert a portion of hydrogen fuel from the main fuel conduit (217) downstream of the heat exchanger (224), and a hydrogen fuel cell (260) configured and arranged to produce electric power using hydrogen fuel diverted from the second fuel offtake (262).