This 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 in targeted radionuclide therapy. The different chapters describe the fundamental concepts of nuclear and hybrid imaging instrumentation and quantitative imaging, internal radiation dosimetry and radiobiology aspects of targeted radionuclide therapy, then discuss the various components and steps required for implementing a dose calculation and treatment planning methodology in targeted radionuclide therapy. Some computer programs are described and illustrated with some useful features and clinical applications. The book is suitable for Scientists working in academic or industrial environments focusing on translational research and therapeutic nuclear medicine and radiology.