Cone-beam computed tomography (CBCT) is an imaging modality that trades the image quality of conventional CT for incredible versatility by capturing an entire volume in a single rotation without requiring table motion. CBCT enables images for diagnostic and interventional radiology as well as in radiation oncology. It provides volumetric imaging mounted to fluoroscopy systems and vertically oriented CT systems suitable for imaging the extremities. Spatial coverage is achieved by a wide-angle X-ray beam and a standard X-ray detector used in fluoroscopy systems. This produces a novel and fascinating modality that must be appreciated for its own strengths and weaknesses. Theory and applications of CBCT detectors are provided in this chapter.