Optical monitoring of living tissues in the near-infrared (NIR) region of the spectrum (700 to 1300 nm) was first demonstrated some 30 years ago by Professor Frans F. Jöbsis of Duke University. Jöbsis had intended to study the oxidation-reduction (redox) behavior of the copper band (CuA) of cytochrome c oxidase (cyt a,a3) to understand certain anomalies in the behavior of the mitochondrial respiratory chain in the ultraviolet and visible regions between living tissue and isolated preparations of mitochondria. Instead, he discovered a new window into the body—for NIR light penetrates deeply into living tissues. Jöbsis's pioneering studies proved it was possible to interrogate hemoglobin absorption and saturation and to assess the redox state of vital organs such as the brain directly through skin and bone. He and his collaborators had also recognized that the tissue hemoglobin signals provided valuable information about the oxygen (O2) content of the tissue, and cyt a,a3 signaled the availability of cellular O2 for oxidative phosphorylation. The ability to noninvasively monitor the O2 delivery-uptake relationship has made NIR spectroscopy a unique tool for the assessment of tissue oxygen sufficiency in health and disease.