This chapter discusses isochronal annealing of electrical resistivity of U (uranium) after different neutron irradiations. Due to the anisotropic mechanical properties of α-U the thermal cycling connected with the isochronal annealing programms leads to irreproducible changes at Δρ and reliable data can only be obtained from single crystals or quasi-single crystals. Positron data on the vacancy formation energy are available for the high temperature phases only, however self diffusion has been investigated for the α-phase. A steep annealing stage I was observed between 7K and 20K after irradiation with 1.7 MeV electrons. However, strong thermal cycling effects obscure these results obtained with polycrystalline samples.