Instrumentation for the continuous measurement of stable isotopes in atmospheric trace substances such as carbon dioxide (CO2), methane (CH4), and also water vapor (H2O), has grown tremendously in the last decade. This is due to the develop- ment of new highly sensitive infrared spectroscopic techniques such as Fourier transform infrared spectroscopy, or laser-based systems like quan- tum cascade laser absorption spectroscopy and cavity ring-down spectroscopy. These systems are not only applied for precise continuous concentra- tion measurements, but have also recently been used for high-frequency isotope measurements of net ecosystem-atmosphere exchange fluxes. Similar development of isotope ratio mass spec- trometry systems led to successful application of the spectrometers in quasicontinuous mode at field stations. The currently available continuous measurement systems for CO2, CH4, and H2O sta- ble isotopic compositions are described here and information about calibration, maintenance, and quality assurance requirements of these systems is given.