Radionuclide or metabolic therapy offers a possibility of targeted therapy with fewer side effects if tumour specific radiopharmaceuticals are developed. Accurate dose planning and correct estimate of the therapeutic effect are then necessary. The objective of individual dose planning is to determine the absorbed dose per unit of activity administered. If this ratio can be accurately calculated, determining the activity to be given to the patient is then possible in order to achieve the required "therapeutic" effect. The book addresses the issue of quantitative SPECT and PET imaging in radiation dose calculations and treatment planning in metabolic radiotherapy with special emphasis on the use of Monte Carlo techniques for radiation transport calculations of photons and beta minus or alpha particles.  The book covers the applications of Monte Carlo (MC) calculations in therapeutic nuclear medicine from basic principles, to computer implementations of software packages and their applications in radiation dosimetry and treatment planning. It is written for nuclear medicine physicists and physicians and can serve as a supplement art text for medical imaging and radiation dosimetry graduate courses in both sciences and medical faculties.