Positron Emission Tomography (PET) combined with Magnetic Resonance Imaging (MRI) is a state-of-the-art hybrid imaging modality that merges PET’s functional imaging with MRI’s exceptional anatomical resolution. This integration enables the simultaneous acquisition of metabolic, structural, and functional data, offering unique diagnostic and research potential. PET/MRI systems are particularly advantageous in oncology, neurology, and cardiology, as they provide high soft-tissue contrast, enhanced lesion detection, and precise localization—with MRI contributing no additional ionizing radiation, making the combined modality comparatively safe. The following chapter explains the fundamentals of PET, including its principles, data corrections, and radiotracer production, such as the radiolabeling with fluorine-18. Coupled with MRI’s imaging basics, the chapter offers background knowledge about in vivo imaging, quantification methods such as standardized uptake values (SUVs), and an introduction to more advanced graphical analysis and compartmental models. The ability to analyze metabolic and functional changes alongside detailed anatomical imaging opens new research opportunities, improves early diagnosis, optimizes treatment planning, and supports therapy monitoring. Despite challenges like high costs and operational complexities, PET/MRI systems continue to evolve, with advancements in hybrid technology and quantification methods pushing the boundaries of personalized and precise medicine. These innovations highlight PET/MRI’s transformative potential, not only in research settings but also in routine clinical practice, paving the way for more effective and comprehensive diagnostic solutions.