Monitoring brain function with light in vivo has become a reality. The technology 33 of detecting and interpreting patterns of reflected light has reached a degree of 34 maturity that now permits high spatial and temporal resolution visualization at both 35 the systems and cellular levels. There now exist several optical imaging methodolo- 36 gies, based on either hemodynamic changes in nervous tissue or neurally induced 37 light scattering changes, that can be used to measure ongoing activity in the brain. 38 These include the techniques of intrinsic signal optical imaging, near-infrared optical 39 imaging, fast optical imaging based on scattered light, optical imaging with voltage 40 sensitive dyes, and two-photon imaging of hemodynamic signals. The purpose of 41 this volume is to capture some of the latest applications of these methodologies to 42 the study of cerebral cortical function. 43 This volume begins with an overview and history of optical imaging and its use 44 in the study of brain function. Several chapters are devoted to the method of intrin- 45 sic signal optical imaging, a method used to record the minute changes in optical 46 absorption due to hemodynamic changes that accompanies cortical activity. Since the 47 detected hemodynamic changes are highly localized, this method has excellent 48 spatial resolution (50¿100 µm ), a resolution sufficient for visualization of fundamen- 49 tal modules of cerebral cortical function.