According to the present invention, a method and apparatus for non-invasively determining the blood oxygen saturation level within a subject's tissue is provided. The method comprises the steps of: 1) providing a near infrared spectrophotometric sensor operable to transmit light along a plurality of wavelengths into the subject's tissue; 2) sensing the light transmitted into the subject's tissue using the sensor, and producing signal data representative of the light sensed from the subject's tissue; 3) processing the signal data to account for physical characteristics of the subject; and 4) determining the blood oxygen saturation level within the subject's tissue using a difference in attenuation between the wavelengths. The apparatus includes a sensor having a light source and at least one light detector, which sensor is operably connected to a processor. The sensor is operable to transmit light along a plurality of wavelengths into the subject's tissue, and produce signal data representative of the light sensed from the subject's tissue. The algorithm is operable to process the signal data to account for the physical characteristics of the subject being sensed.