Tissue oximeters using near-infrared spectroscopy (NIRS) have been applied for evaluating the viability of cerebral tissue in surgery and for monitoring muscle oxygenation in sport medicine and rehabilitation. We developed NIRS devices for muscles and the brain to measure deep and superficial tissues as well as a finger-mounted oximeter for the fetus and a neonate by miniaturizing the optical probe and analyzing light propagation in the layered tissue model. Various applications are also progressing to evaluate the flaps oxygenation and observe cerebral cortex activity. We outlined the principle of oximetry, its medical device development, and its applications.