Nuclear medicine has evolved from a niche diagnostic discipline into a cornerstone of modern precision healthcare. By integrating molecular imaging, quantitative diagnostics, radiopharmaceutical sciences, and targeted radionuclide therapy, contemporary nuclear medicine provides unparalleled opportunities for disease detection, characterization, treatment planning, and therapeutic monitoring. This book presents a comprehensive exploration of the scientific foundations, technological innovations, clinical applications, and future directions of nuclear medicine. It bridges fundamental concepts in radiation physics, radiobiology, and radiopharmaceutical chemistry with advanced imaging technologies such as SPECT, PET, PET/CT, and PET/MRI. Furthermore, it addresses emerging fields including theranostics, artificial intelligence, quantitative imaging, molecular medicine, nanotechnology, and next-generation autonomous imaging systems. Designed for physicians, medical physicists, radiopharmacists, biomedical engineers, researchers, and graduate students, this volume provides both theoretical foundations and practical clinical perspectives. Through a multidisciplinary approach, the book aims to support education, research, innovation, and clinical excellence in the rapidly evolving landscape of molecular imaging and precision oncology.