1. A spectrophotometric apparatus for determining the glucose concentration in body tissues transcutaneously and non-invasively, comprising: (a) a directional optical light source located external to the body, the spectral composition of the beam of light from said source being such that it can penetrate the skin to tissues below; (b) means for collecting light transmitted or diffusely reflected from said irradiated tissue; (c) means for detecting and converting into electrical signals light gathered from at least one band with a measuring signal wavelength .lambda.G of 1575, 1765, 2100 or 2270 + or - 15 nm, typical of the glucose absorption spectrum, and at least one band with a reference signal wavelength .lambda.R in the range of 1000 to 2700 nm, typical of the absorption spectrum of background tissue containing glucose but in which the absorption of glucose is nil or insignificant; and (d) means for transforming said electrical signals into data representing glucose determinations.