The present volume Uranium C5 covers the physical properties of U0 - the production 2 and preparation of U0 were already treated in Uranium C4, whereas the chemical proper 2 ties will be the subject of the forthcoming part C6. U0 is the most important chemical compound in all aspects of nuclear technology. 2 It is and will be for the foreseeable future the fuel for all light and heavy water reactors as well as (in the mixed crystal with Pu0 ) for the fast breeder reactors. Therefore, the 2 nuclear engineer has to understand the behavior of U0 under all conditions existing during 2 operational (and possibly failure) states of a nuclear reactor, e. g., not only in the solid state but also to some extent in the liquid and gaseous states. Besides high scientific interest in the sometimes unique or unusual properties, e. g., at low temperatures, a lot of data and physical properties which are critical for its use as a nuclear fuel have been determined more or less accurately. Creep, swelling, irradiation densification, and fission gas behavior in the fuel are properties which have been evaluated up to the high temperatu res (near the melting point) which may exist in U0 fuel due to its low thermal conductivity. 2 Besides these more technical data there have been accumulated a lot of important physical data, e. g.