Boron is the main p-type dopant in Si and in Ge and its diffusion is strictly related to the purity of the hosting lattice since it is dominated by the point defects in both the cases. Actually, the atomistic mechanism of B diffusion has been revealed to different extents in the two cases, since in Si it has been widely investigated for more than 40 years attaining a very deep comprehension, while in Ge it is the subject of a currently expanding scientific debate. Recently, the experimental data and the associated picture of B transport in the two materials are disclosing many common aspects and similarities, some of which will be reported in this talk.