State of the art nanoelectronic devices are mainly fabricated on silicon (Si). In order to take advantage of the properties of germanium (Ge) in advanced electronic nanodevices all steps of device fabrication must be controlled. This, in particular, concerns the diffusion, doping, and activation of dopants in Ge. In this paper, the mechanism of self- and dopant diffusion in Ge under thermal equilibrium and non-equilibrium conditions are reviewed. Non-equilibrium conditions can, e.g., be realized by irradiation, implantation, and the dissolution of unstable defect clusters. Defect reactions mediating dopant diffusion and dopant deactivation in Ge under different experimental conditions are discussed. Special attention is paid to the charge states of the involved point defects and their Coulomb interactions that explain numerous results on dopant diffusion and dopant deactivation. Controlling the formation of vacancies V and self-interstitials I is the key to develop successful defect engineering strategies that are required to achieve the objectives for the fabrication of Ge-based devices.