The noninvasive method of functional near-infrared spectroscopy (fNIRS) has recently been used to measure changes of optical parameters such as light absorption of brain tissue. Hemodynamics and oxygenation changes related to brain activation can be evaluated using fNIRS, with the most widely evaluated parameters being oxygenated hemoglobin (Oxy-Hb), deoxygenated hemoglobin (Deoxy-Hb), as well as total hemoglobin, which is the sum of Oxy-Hb and Deoxy-Hb and which reflects cerebral blood flow. The typical response is an increase in concentrations of Oxy-Hb and Total-Hb accompanied by a decrease in Deoxy-Hb; however, a variety of oxygenation and deoxygenation changes have been found, particularly regarding clinical studies and developmental studies of infants and older adults. The findings of increases of Deoxy-Hb as well as decreases of Oxy-Hb and Total-Hb are important for understanding clinical and development changes. In addition, the findings suggest possible limitations or problems with blood oxygenation level dependent (BOLD)-fMRI for clinical and developmental studies, and for revealing the complexity of hemodynamics and oxygenation responses. (C) 2001 Society of Photo-Optical Instrumentation Engineers.