The invention relates to an apparatus for non-invasive in vivo measurement by Raman spectroscopy. Blood glucose concentration is measured in vivo using Raman spectroscopy in such a way as to receive at a detector (229) light scattered from glucose in interstitial fluid in skin in a measurement location (217) at a depth (218) of from 200-300 mum below the skin surface providing improved retention of correct calibration and transferability of calibration between individual subjects.