Abstract: An apparatus and method for transcutaneous measurement of a substance within a body. Transcutaneous measurement of substances within a subjects' tissues or fluids is effected through the use of the present invention. A light source, focused by an optical system, directs light towards a subject. Light returning from the subject as a result of this illumination, is collected, focused and directed to a detection system. The output of this detection system can then be analyzed to determine the concentration of the substance being measured. In a preferred embodiment bilirubin is measured. However, the measurement of substances such as oxygen, carbon dioxide, glucose, albumin, hemoglobin and cholesterol, among others, is also contemplated.