My dissertation targets novel advancements for quantitative molecular imaging and therapeutics. The field is experiencing a renaissance due to new technologies and advancement of paradigm-breaking approaches to molecularly targeted radiation therapy. My work simulates and validates dosimetric properties of therapeutic radionuclides, and performance measurements of PET scanners, including extended axial-field-of-view PET scanners. The dosimetry work enhances accuracy of clinical radionuclide dosimetry. The PET scanner simulation predicts performance of novel PET scanners to expand research boundaries.