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 (10(5) s/pointing, limiting N-HI similar to 10(18) Cm-2) and a deep patch (10(4) s/pointing, limiting N-HI N similar to 3 x 10(18) Cm-2) in carefully selected areas, and we outline the case to do this.