The development of X-ray computed tomography (CT) has been founded on the discovery of X-rays, the inception of the Radon transform, and the growth of X-ray digital data acquisition systems and computer knowledge. Dissimilar conventional X-ray imaging (general radiography), CT reconstructs cross-sectional anatomical images of the internal structures according to X-ray attenuation co-efficients (approximate tissue density) for almost every region in the body. This articleappraisals the essential physical principles and practical aspects of the CT scanner, including numerous notable evolutions in CT technology that resulted in the appearance of helical, multidetector, cone beam, portable, dual-energy, and phase-contrast CT, in integrated imaging modalities, such as positronemission- tomography一CT and single-photon-emission-computed-tomography-CT, and in clinical applications, including image acquisition parameters, CT angiography, image adjustment, versatile image visualizations, volumetric/surface rendering on a computer workstation, radiation treatment planning, and target localization in radiotherapy. The understanding of CT characteristics will provide more effective and accurate patient care in the fields of diagnostics and radiotherapy, and can lead to the improvement of image quality and the optimization of exposure doses.