Isotopomeric molecules show the same chemistry. Via their different physical properties (different atomic mass values and nuclear magnetic moments) the structure and function of (bio)molecules can be elucidated at the atomic level without perturbation using NMR and vibrational techniques. The synthesis of site-directed isotopically enriched systems in comparison with normal organic synthesis will be discussed. Other topics will be: how can the various isotopic sensitive physical techniques be used to obtain this exquisite structural and functional information at the atomic level.