It was recalled that, in Ge, vacancies had historically been believed to dominate most diffusion-related phenomena such as self-diffusivity or impurity migration. This was contrasted with the case of Si, where self-interstitials also played decisive roles, despite the similarities in the chemical nature of both materials. Density functional calculations of the formation and properties of vacancy–donor complexes in Ge were reported. It was predicted that most vacancy–donor aggregates were deep acceptors, and together with their high solubilities, it was concluded that they strongly contributed to inhibiting donor activation levels in Ge. It was recalled that, in Ge, vacancies had historically been believed to dominate most diffusion-related phenomena such as self-diffusivity or impurity migration. This was contrasted with the case of Si, where self-interstitials also played decisive roles, despite the similarities in the chemical nature of both materials. Density functional calculations of the formation and properties of vacancy–donor complexes in Ge were reported. It was predicted that most vacancy–donor aggregates were deep acceptors, and together with their high solubilities, it was concluded that they strongly contributed to inhibiting donor activation levels in Ge.