Voltage-sensitive dye imaging (VSDi) enables visualization of information processing in different areas of the brain with reasonable spatial and temporal resolution. VSDi employs different chemical compounds to transduce neural activity directly into the changes in intrinsic optical signal. Physically, voltage-sensitive dyes (VSDs) are chemical probes that reside in the neural membrane and change their fluorescence or absorbance in response to membrane potential changes. Based on these features, VSDs can be divided into two groups-absorbance and fluorescence. The spatial and temporal resolution of the VSDi is limited mainly by the technical characteristics of the optical imaging setup (e.g., computer and light-sensitive device-charge-coupled device (CCD) camera or photodiode array). In this article, we briefly review the development of the VSD, technique of VSDi and applications in functional brain imaging.