Background. In the 1990s, following years of development of meteorological data assimilation by the Numerical Weather Prediction (NWP) community, the data assimilation methodology began to be applied to constituents, with a strong focus on stratospheric ozone (Rood 2005; Lahoz et al. 2007a). Because of its comparatively later application, constituent data assimilation is less mature than meteorological data (henceforth NWP) assimilation. Nevertheless, there has been substantial progress over the last 15 years, with the field evolving from initial efforts to test the methodology to later efforts focusing on products for monitoring ozone and other constituents. More recently, the production of ozone forecasts by a number of operational centres has become routine. A notable feature of the application of the data assimilation methodology to constituents has been the strong interaction between the NWP and research communities, for example, in the EU-funded ASSET project (Lahoz et al. 2007b). A list of acronyms can be found in Appendix.