Associated with changes in the level of physiological activity in neuronal tissue are changes in the intrinsic optical properties of the tissue. As a consequence, it is possible to optically monitor neuronal activity without the use of dyes or other contrast-enhancing agents. Such optical techniques have been applied in the laboratory for more than 50 years. Recent developments in near-infrared spectroscopy and intraoperative optical imaging have suggested a number of clinically important applications of this technology. This article provides an overview of what is known about the physiological correlates and underlying mechanisms associated with activity-evoked optical changes in neuronal tissue.