Implantable multi-wavelength oximetry sensors, which can be used to monitor a patient's blood oxygen saturation level, are described. The sensor includes an implantable sensor housing within which are located a plurality of light sources that each transmits light of a different wavelength. Also within the sensor housing are one or more surfaces that are configured to combine the light from the plurality of light sources into a beam of light for transmission through a portion of the housing (e.g., a window) through which light can exit and enter the housing. Additionally, a light detector is located within the sensor housing, to detect light scattered by blood back into the housing. This description is not intended to be a complete description of, or limit the scope of, the invention. Other features, aspects, and objects of the invention can be obtained from a review of the specification, the figures, and the claims.