In this publication the basic principles of radiation physics, imaging and non-imaging instrumentation used, measurement of the administered activity, calibration procedures and methods for obtaining quantitative information on the biodistribution of the radioactive drug to be used with radioisotopes relevant to therapy are specified. It also describes methods for segmentation and registration of images acquired at different time points, strategies for fitting and integration of activity measurements over the time of treatment, absorbed dose calculations and derived dosimetric indexes with methods to estimate the overall uncertainty of different radionuclide therapies. The aim of this book is to fill the existing gaps in education and training of medical physicists on methods for patient-specific dosimetry. The overall objective of this book is to highlight the tools and methodologies to assure that radiopharmaceutical therapy is implemented through a dosimetry-guided individualized treatment approach.