A FRAMELESS COMPUTERIZED navigating system was successfully employed in 20 cases of open microsurgical operations on deep-seated brain tumors and vascular malformations. Localization in space was made by measuring the traveling time of sonic waves (24 kHz) in air. This allowed the construction of mechanically simple, lightweight freehand targeting instruments. The tips can be localized with a measuring accuracy of ± 1 mm intracranially on the basis of computed tomographic scans. This thoroughly redesigned sonic stereometrical device is being optimized for everyday use; magnetic resonance imaging, digital substraction angiography, and other diagnostic modalities will be implemented. Some perspectives on “computer-aided (neuro)surgery” are discussed.