The experience of external radiotherapy can only to a limited extent be used to understand therapeutic effects of radionuclide therapy. A major difference is that the dose-rate at radionuclide therapy is at least two orders of magnitude lower. Part of this chapter deals with estimates of the necessary dose-rate and exposure time in combination in order to deliver therapeutic effects to tumour cells. It is proposed that combinations of about 0.1–0.2 Gy/h for several days or about 1 Gy/h for at least 1 day is necessary. Such dose-rates can be achieved with the help of cross fire radiation. Effects of radionuclide therapy in terms of apoptosis, cell-cycle blocks and hyperradiosensitivity are also discussed.