A method and device are provided for the determination of the concentration of compounds in body tissue. The method utilises optical methods based on the interaction of light with compounds, whereby the concentration of the compound under analysis is determined. The method is especially suited for analysis for the concentration of glucose in blood or tissue of diabetic patients, a device being implanted underneath the skin of the patient and the method being carried out by using the implanted device. The device contains photo detectors at different levels connected by wires to an electronic circuit device. A differential analysis is performed on the signals from the detectors to reduce the effect of skin on the analysis.