The fundamental goal of computer-assisted surgery is to make orthopaedic surgery patient-specific, minimally invasive, and quantitative. The components of computer-assisted surgery include preoperative imaging and planning, intraoperative execution, and postoperative evaluation. Ideally, these components are integrated such that sophisticated diagnostic technologies are used to create a patient-specific surgical plan. This plan is then programmed into a computer-assisted intraoperative system so that it can be precisely executed. Finally, the patient outcome is tracked longitudinally in a quantitative fashion. Computer-assisted surgery relies on the use of quantitative data rather than surgeon feel and intuition to facilitate clinical decision-making. As surgeons rely more on quantitative feedback, they must establish appropriate specifications for various operations. These specifications should be clinically relevant and must have known targets and tolerances. This overview provides examples of quantitative surgery as applied in navigated total knee replacement and anterior cruciate ligament reconstruction and in the more recent indication of robotic unicondylar knee replacement. Computer-assisted surgery represents a set of tools that facilitate quantitative surgery. To effectively use these tools, however, one must identify technical specifications that are clinically relevant for the various procedures; these specifications must be associated with known target values and tolerances and must have the capability of being reliably measured by computer-assisted surgery tools. Clinical and basic-science research is necessary to better define technical specifications for navigated procedures.