21 CM searches for DIM galaxies

We review very strong selection effects which operate against the detection of dim (i.e. low surface brightness) galaxies. The Parkes multibeam instrument offers a wonderful opportunity to turn up new populations of such galaxies. However, to explore the newly accessible parameter space, it will be necessary to survey both a very deep patch (10⁵ s/pointing, limiting N _hi ∼ 10¹⁸ cm^-2) and a deep patch (10⁴ s/pointing, limiting N _hi ∼ 3 × 10¹⁸ cm^-2) in carefully selected areas, and we outline the case to do this.