Positron emission tomography combined with computed tomography (PET/CT) is a hybrid imaging modality that integrates molecular sensitivity with anatomical localization, enabling the in vivo characterization and quantification of physiological and pathological processes. This chapter provides a structured overview of PET/CT principles, beginning with the nuclear and particle physics underpinning positron-emitting radionuclides and β+ decay, including positron transport, annihilation photon production at 511 keV, and the key physical limitations imposed by positron range and photon non-collinearity. The detection chain is then described from coincidence event formation and line-of-response definition to scintillation detector design, photodetector technologies (including silicon photomultipliers), energy discrimination, timing performance, and the rationale for time-of-flight PET. A dedicated section addresses image reconstruction, contrasting analytical approaches with modern iterative statistical methods (MLEM/OSEM) and extending to penalized-likelihood and Bayesian frameworks for noise control and resolution recovery. The chapter then focuses on the corrections required for quantitative fidelity, including attenuation correction (with emphasis on CT-derived μ-maps and the specific challenges of PET/MRI), scatter and randoms estimation, normalization, dead-time, and calibration stability. These elements are linked to quantitative endpoints, with standardized uptake value (SUV) used as a practical clinical metric and discussed in terms of biological and technical determinants of repeatability (uptake time, glycaemia, protocol harmonization, and reconstruction settings). Finally, dosimetric considerations are summarized within a radiological protection framework, addressing patient exposure from both radiopharmaceutical administration and CT acquisition, and outlining optimization strategies consistent with ALARA principles. The chapter concludes with advances shaping contemporary PET, including digital detectors, total-body PET, AI-enabled reconstruction and workflow automation, and theranostic paradigms supporting individualized imaging and treatment planning. Overall, the chapter aims to provide a coherent conceptual foundation for critically understanding PET/CT performance, limitations, and quantitative interpretation in clinical and research practice.