Image-guided interventions using intraoperative three-dimensional (3D) imaging can be less cumbersome than systems dependent on preoperative images, especially by needing neither image-to-patient registration nor a lengthy process of segmenting and generating a 3D model. In this dissertation, a method for computer-assisted surgery using direct navigation on intraoperative images is presented. In this system the registration step of a navigated procedure was divided into two stages:​ preoperative calibration of images to a ceiling-mounted optical tracking system, and intraoperative tracking during acquisition of the 3D image. The preoperative stage used a custom-made multi-modal calibrator that could be optically tracked and also contained fiducial spheres for radiological detection;​ a robust registration algorithm was used to compensate for the high false-detection rate that arose from the optical light-emitting diodes. Intraoperatively, a tracking device was attached to bone models that were also instrumented with radio-opaque spheres;​ a calibrated pointer was used to contact the latter spheres as a validation. The fiducial registration error of the calibration stage was approximately 0.1 mm with the Innova 3D X-ray fluoroscope and 0.7 mm with the mobile-gantry CT scanner. The target registration error in the validation stage was approximately 1.2 mm with the Innova 3D X-ray fluoroscope and 1.8 mm with the mobile-gantry CT scanner. These findings suggest that direct registration can be a highly accurate means of performing image-guided interventions in a fast, simple manner.