Computer-assisted surgery has been advocated as a significant enabling technology that will enhance the surgical technique of various orthopaedic procedures. The computer becomes a sophisticated measuring tool, determining the three-dimensional spatial orientation of fiducial points, which may be established by a variety of referencing methods. These fiducial points or arrays may define a bone, an instrument, or a prosthesis. Current referencing methods include using segmented computer tomograms; fluoroscopic images; ultrasound images; and imageless, direct anatomic point-picking methods. Tracking technologies use optical cameras and electromagnetic coils. Optical systems have high reliability with errors of less than 0.5 mm. Electromagnetic trackers have a similar capability, but are less reliable because of the distortion of the electromagnetic signal that may result from the complex operating room environment. Accuracy with current CT-referenced systems approximates 1 degrees or 1 mm. Other methods such as fluoroscopy or ultrasound are less precise because of difficulty related to the ability to consistently define a specific anatomic structure. Descriptive measures of outcome include standard deviation and quantification of error. Process capability indices or Six Sigma are suitable methods for comparing outcomes with computer-assisted surgery and can be generalized from various approaches.