Water vapour is a key climate variable since it accounts for about 60% of the natural greenhouse effect for clear skies (Kiehl and Trenberth, 1997) and provides the largest positive feedback in model projections of climate change (Held and Soden, 2000). During the 1980s and 1990s, stratospheric water vapour (H₂O) was observed from balloon measurements over Boulder to increase with a trend of 1-1.5 %/yr (Oltmans et al., 2000). This data record was extended to present and re-assessed recently (Hurst et al., 2011) with the result, that there was not one trend over the 30-years period, but at least 4 period of increasing and decreasing water vapour. The sudden drop in stratospheric water vapour which occurred in 2001 is still not fully understood. Rosenlof and Reid (2008) have shown that sea surface temperatures may play a relevant role. Solomon et al. (2010) demonstrated the importance of stratospheric water vapour for the radiative forcing, also showing that the drop in 2001 affected the latter noticeably. The small-scale processes ruling the H₂O transport through the tropical troposphere and the related dehydration mechanisms are still under debate; in particular, the role of deep convective clouds is under discussion (e.g., Schiller et al., 2009; Flury et al., 2011; Dessler, 2009). Recently, several data records from satellites on HDO have become available. Since isotopologues of water vapour have the advantage to keep a phase transition record of the water vapour in the observed air mass, they can be used to investigate the role of phase transitions for the water vapour transport through the tropopause. Steinwagner et al. (2010) have shown that the stratospheric HDO-to-H₂O ratio is close to the value which would be expected for pure Rayleigh fractionation during the transport, indicating that in situ formation of cirrus clouds (“freeze-drying”) during uplift is the dominating process, however small but significant deviations hint also towards lofting of ice particles into the stratosphere as a minor but still relevant contribution. Further analysis of water vapour isotopologue data sets is very promising. Methane abundances, as an important source of water vapour in the upper stratosphere, have been observed to stagnate over a decade or so, before they started to increase again, and stronger as before (Rigby et al., 2008). Neither the stagnation nor the subsequent stronger growth has been fully understood to date. Analyses of the impact on the stratospheric water vapour budget are not available as well. An open question is how methane will increase in future, and how the methane oxidation efficiency will evolve in future due to changing atmospheric composition.