Frame-based stereotactic systems provide valuable localization information for the performance of neurosurgical procedures. State-of-the-art systems provide for sophisticated preoperative planning and intraoperative interactive help with resection of predefined tissue volumes, providing assistance with resection of intrinsic brain tumors whose margins may not be readily visible at the time of surgery. There remain, however, a majority of neurosurgical procedures that would benefit from some form of localization but for which the application of a cumbersome frame and arc system is inconvenient. Frameless stereotactic localization or "cranial-based" localization provides a rapid and convenient means for computer-interactive localization and surgery for many of these cases. The stereotactic operating arm system is designed to complement frame-based stereotactic surgery, bringing standard neurosurgical procedures into the realm of computer-assisted interactive localization. The pace of progress is such that eventually, some form of computer interactive cranial localization will become a common neurosurgical tool. The concept of frameless localization is taken a step further in the frameless radiosurgical system described above. It is reasonable to think that this concept of "automated feature recognition," whether for recognizing scalp or bone contours, will ultimately become the foundation of procedures requiring cranial localization with reference to previously obtained digital images.