A stereotactic system is described. It is built around a baseplate which is fastened to the head of the patient and to various diagnostic and therapeutic modalities. In principle this system can function as an interface between most diagnostic (CT scanners, PET scanners, conventional X-ray instrumentation for angiograms and pneumoencephalograms) and therapeutic (stereotactic instruments, linear accelerators) instruments. It has been in practical use since 1976 and used in a number of ‘diagnostic-therapeutic’ combinations.