Water vapour, a strong greenhouse gas, plays an important role for the dynamics of the middle atmosphere. As a reservoir of chemically active HOx radicals, it is also linked to many chemical processes like the natural destruction of ozone. In order to improve our knowledge of the amount and variability of water in the middle atmosphere, the Sub-Millimetre Radiometer (SMR) on board the Odin satellite, launched in February 2001, observes several thermal emission lines of water vapour in the 485-580 GHz spectral range from the Earth's limb. Bands around 489.9 and 490.4 GHz are used to study water vapour and its isotopes, on the basis of four observation days per month. Vertical profiles of H2O-16, H2O-18, and HDO are retrieved between roughly 20 and 70 km in the stratosphere and mesosphere. A strong water vapour line at 557 GHz is simultaneously measured in a second band, providing information in the mesosphere and lower thermosphere between about 40 and 100 km. Measurements of H2O-17 at 552 GHz in monthly intervals complete the picture of middle atmospheric water vapour provided by Odin. The measurements of the isotope HDO in the 20-70 km altitude range allow to study the isotopic depletion of water, potentially supplying information on the origin of stratospheric water vapour: transport of tropospheric air through the tropical tropopause layer (TTL) vs in-situ chemical production by methane oxidation. The unique measurements of the isotopes H2O-18 and H2O-17 may provide a crucial test for our understanding of the complex mass-dependent and mass-independent chemical reaction mechanisms controlling exchange of heavy isotopes of oxygen.Read less <