There is a need for frameless guidance systems to aid neurosurgeons in planning the exact location of a craniotomy, to define the margins of tumors and to precisely locate neighboring critical structures. We have developed an automatic technique for registering clinical data, such as segmented MRI or CT reconstructions, with the actual head of the patient on the operating tsble. A second method calibrates the position of a video camera relative to the patient. The combination allows a visual mix of live video of the patient with the segmented 3D MR/or CT model. This registration enables us to employ enhanced reality techniques for planning and guiding nellrosurgical procedures by merging live video images with the 3D reconstructions, and to interactively view extracranial or intracraniai structures in a non-intrusive manner.