Computer-assisted surgery (CAS) uses computer technology to help guide surgical procedures and has evolved from early systems that located brain tumors to current navigation systems that provide real-time positional information of surgical tools to help surgeons accurately reach anatomical targets and optimally position implants while avoiding areas of risk. CAS systems can be passive with just navigation, semi-active assisting with guide tools but not surgery, or active performing pre-programmed surgical actions. Modern navigation relies on tracking reflective markers in real-time rather than external imaging to construct a 3D model of the patient's anatomy.