Isotopic ratios provide a powerful method to study the origin of elements and the chemical evolution of the Universe. Spectroscopic analysis is required for any remote astronomical source, with limits on angular resolution, sensitivity, and knowledge of the physical conditions. Molecular lines are particularly well suited as the spectral signatures of different isotopologues are well separated in frequency. I will overview the available spectroscopic observations of isotopic molecular substitutes involving D, 13C and 15N in galactic and extragalactic interstellar clouds as well as in the Solar System. The chemical connection between these various environments will be discussed at the light of observed isotopic ratios and the microscopic processes at work.