This chapter discusses the results of the several studies which were made, mostly under NASA sponsorship, but supplemented by work independently funded by Lockheed-California Company, which greatly added to the store of credible knowledge on the subjects. It provides detailed information about some of the principal elements of liquid hydrogen (LH2)-fueled aircraft fuel system, using the reference 400-passenger transport aircraft as a basis for illustration. The final design of 400-passenger, LH2-fueled aircraft from the early studies. In purpose and basic function these systems for LH2-fueled aircraft are identical with those of conventionally fueled aircraft to supply fuel according to a specified flow/pressure schedule to the engines. It describes the general characteristics and requirements of a representative LH2-fueled aircraft fuel system. Weight estimating relationships normally used for conventional subsonic passenger transport aircraft were employed, except as it was necessary to modify them to account for features associated with use of LH2 fuel.