  Theoretical galaxy formation models are an established and powerful tool for
interpreting the astrophysical significance of observational data, particularly
galaxy surveys. Such models have been utilised with great success by optical
surveys such as 2dFGRS and SDSS, but their application to radio surveys of cold
gas in galaxies has been limited. In this chapter we describe recent
developments in the modelling of the cold gas properties in the models, and how
these developments are essential if they are to be applied to cold gas surveys
of the kind that will be carried out with the SKA. By linking explicitly a
galaxy's star formation rate to the abundance of molecular hydrogen in the
galaxy rather than cold gas abundance, as was assumed previously, the latest
models reproduce naturally many of the global atomic and molecular hydrogen
properties of observed galaxies. We review some of the key results of the
latest models and highlight areas where further developments are necessary. We
discuss also how model predictions can be most accurately compared with
observational data, what challenges we expect when creating synthetic galaxy
surveys in the SKA era, and how the SKA can be used to test models of dark
matter.
