Near infrared spectroscopy has recently been used to measure changes of optical parameters (i.e., light absorption or scattering) of brain tissue. The fact that the equipment is generally compact, portable, noninvasive, and reasonably priced makes it ideal for clinical and nonclinical evaluation and monitoring of brain function Clinical and nonclinical studies evaluating changes related to light absorption are discussed, with an emphasis on cerebral blood oxygenation (CBO) changes and hemodynamic responses while performing cognitive tasks. With respect to the clinical studies, the focus is on variations in patterns of oxygenated hemoglobin (Oxy-Hb), deoxygenated hemoglobin (Deoxy-Hb) and Total-Hb (sum of Oxy-Hb and Deoxy-Hb). The studies about clinical applications includes research we have conducted with older adults and aphasics. Implications regarding the use of MRS for clinical purposes are considered.