The analysis of stable isotope ratios of hydrogen and oxygen in water can be applied in many research fields. This thesis starts, after some general information about the stable isotopes of water with introducing our measurement techniques. We utilized a laser-based, optical method, which has profound advantages compared to the traditional analysis method using Isotope Ratio Mass Spectrometry (IRMS). Then, three applications are demonstrated. The first is a novel, accurate method to determine the so-called ice-liquid isotopic fractionation factor, which is one of several important quantities needed for the use of isotopes in studying the world-wide water cycle. The second application is for the isotope analysis of water and ethanol in wine for authenticity investigations. We used the optical analyser with an advanced correction for the other constituents in wine, a method that is faster and cheaper than the official IRMS methods to verify for example the origin of the wine. We also studied the possibility of replacing the use of yet another, complicated method (Nuclear Magnetic Resonance) for wine authenticity. The third application is focused on energy expenditure research, in our case on mice. Using our optical measurements, we could extend this method by using in addition the second rare oxygen isotope (turning Doubly labelled into Triply labelled), and we showed that this extension made the analysis more robust and more accurate. Even more, it enabled us to pinpoint the possible cause of the difference between energy expenditure measurements using the DLW (now TLW) method and indirect calorimetry.