The idea that neurosurgeons can do neurosurgery without being physically present in the operating room is no longer science fiction. With the development of computer—assisted neurological surgery techniques and computer networking capabilities, especially with the upcoming information highway system, tele—presence neurosurgery becomes increasingly realistic. By tele—presence we mean surgeons can remotely monitor, guide, or even do operations while far away from the operating room and the patient (Fig. 1) . Remote monitoring, instrument guiding, and operating are based on a virtual environment (virtual reality) , the environment present in the operating room. Therefore it is also necessary to construct a "virtual patient" for the surgeons at the remote workstation to monitor during surgical procedures. With a passive tele—presence system, neurosurgeons remotely monitor and guide surgery but cannot control the surgical instrument actively. The surgery is done by an assistant surgeon inside the operating room. With an active tele— presence system, a neurosurgeon at a remote site can actively control the movement of surgical instruments. Current medical imaging technology together with correlation strategies, are the foundation of building the virtual patient. Biornedical image visualization as well as surgery planning system offer great surgeon—machine interfaces. Tele—presence can be both passive and active. Tele—presence in neurological surgery has important applications also in education and training. And with the improvement of national information highway system, tele—presence neurosurgery definitely will play an very important role in the medical information highway system. The first part of this paper discusses in detail the fundamental theory involved in a tele—presence surgery system. The discussion centers around the issue of system architecture (software module, hardware setup) , and detailed functionalities for each module are also discussed. The second part of the paper discusses a prototype system in development at Wayne State Universtty Medical Center. The experimental results are also given in this part.