A recent consensus within an international society for sports traumatology revealed that approximately 40% of ACL grafts are being surgically misplaced in current clinical practice. To help solve this problem, a computer-assisted system has been developed at the M.E. Müller Institute for Biomechanics to perform intraoperative planning and guidance of ACL replacement. Dynamic reference bases are fixed on the femur and tibia to track the knee's movement. No intraoperative imaging is required, and potential ligament attachment sites can be directly digitized using a computerized palpation hook in a minimally invasive fashion when used in conjunction with standard endoscopic tools. The palpation hook can be used by the surgeon to interactively define various anatomical structures and reference landmarks that are important for proper ligament positioning. The system can input a standard diagnostic X-ray (sagittal view of the femur) and allows intraoperative registration of this image with the patient to provide valuable X-ray landmarks for intraoperative guidance. The computer helps in situ planning of ligament placement by providing the surgeon with a 3D overview of the relevant anatomical landmarks and information on graft impingement and elongation for various simulated surgical insertions and graft sizes. After planning, the computer helps guide placement of the chosen insertion tunnels. This approach provides an augmented 3D view of knee anatomy and ligament function prior to drilling that is not possible with current procedures. The flexibility of the system in permitting surgeon-defined landmarks and free interpretation of functional factors allows it to support a variety of surgical workflows and techniques.