This chapter offers a comprehensive reference for Computed Tomography (CT), encompassing its historical, technical, clinical, and safety dimensions. Beginning with the pioneering contributions of Allan Cormack and Godfrey Hounsfield, it provides the evolution through successive CT generations, from first to fifth, including modern innovations like spiral/helical CT, multi-slice CT (MSCT), and photon-counting CT. Then moves to the detailed discussion of CT scanner principles, covering Hounsfield units, X-ray attenuation, and image reconstruction through Beer-Lambert’s law. The instrumentation section explains gantry mechanics, detectors, slip rings, and data acquisition systems, while outlining complex image artefacts (e.g., beam hardening, motion, spiral and cone-beam artefacts). The chapter also explains CT quality control, including acceptance testing, routine QC protocols, and dosimetry, with tools such as CTDIvol, DLP, and SSDE, referencing AAPM and ICRU guidelines. Reconstruction methodologies range from traditional filtered back projection to deep learning reconstruction (DLR), detailing CNN architectures and their clinical potential. Further, it addresses contrast enhancement, distinguishing between iodinated, gadolinium, and barium-based agents, and their safe usage. Applications span CT fluoroscopy, cardiac CT, angiography, and virtual endoscopy, supported by a discussion on digital image processing, compression, and matrix resolution. The chapter concludes with guidance on CT room design, radiation shielding, patient safety, dose optimization, and pediatric-specific protocols, ensuring compliance with regulatory standards and workflow efficiency.