Functional mapping techniques including functional magnetic resonance imaging (fMRI), positron emission tomography (PET), and magnetoencephalography (MEG) can be used to study the function of the nervous system. Optical imaging is an emerging technique for functional imaging of the nervous tissue. Functional optical imaging can be classified into two major streams; intrinsic signal optical imaging (ISO) and voltage-sensitive dye optical imaging (VDO). ISO is related to hemodynamic changes such as hemoglobin concentration and oxygenation changes, cytochrome oxidation change, and light scattering. On the contrary, VOD measures changes in membrane potentials of neural cells. Therefore, ISO reflects metabolic activity of neurons, while VOD directly reflects neural activity. Recent advances in optical imaging opened the possibility of its application to clinical situations as well as basic researches. Further, development of optical imaging may greatly contribute to the understanding of the function of the nervous system.