Diagnostic X-ray computed tomography is the workhorse of the radiologist. Although the technology is mature, significant new developments have been taken place during the last years. This lecture gives basics about the CT technology, including hardware, image reconstruction, image quality and dose, and it highlights features of the latest CT systems. Finally differences to other medical CT systems, such as micro CT, C-arm CT, cone-beam CT for image guided radiation therapy or dental volume tomography are discussed.