The main objective in this research project is to develop a non-invasive reflectance oximeter for the measurement of hemoglobin oxygen saturation in peripheral tissues. The principle here is based upon the measurement of tissue reflectances at 5 different wavelengths at the red and near infrared regions. Because the reflectances at these wavelengths vary with oxygen saturation due to the absorption characteristics of hemoglobin and oxyhemoglobin of blood, measurement of these reflectances can be used to estimate hemoglobin oxygen saturation in tissue. A prototype instrumentation system employing solid state electronics such as light emitting diodes and PIN photodiodes has been designed and constructed. The instrument has been shown to give reliable measurement in whole blood. A simple homogeneous tissue model has been established to give simulated tissue reflectances at different oxygen saturation and experimental results were found to correlate well with theoretical calculations using the photon diffusion theory. In vivo tissue reflectance measurements have also been obtained using an isolated gut preparation in dogs. Our experimental results show that reflectances at the red wavelengths give good qualitative correlation with blood oxygen saturation in the tissue. Current effort is aimed at the further development of the photon diffusion theory for the direct estimation of hemoglobin oxygen saturation in tissues.