Once halo parameters are derived, what can one do with them ? Scaling relations between the halo parameters have been reviewed by Kormendy (1988), partly on the basis of the ``no m = 1'' models and results of Athanassoula et al. (1987). Apart from relatively obvious correlations, e.g. between Vmax, disc and the velocity dispersion of the halo, there are a few which are intruiging, such as the (weak) relation between the ratio of halo core radius and optical radius with Hubble type, and the correlation between halo core radius and central density of the halo for Sc - dwarf galaxies. These relations will be further investigated (Kormendy, private communication).
Bosma (1991) shows that the frequency of warps, which is at least 50% for all spirals, depends on the ratio of halo core radius to optical radius of the galaxy : galaxies for which this ratio is small do not have warped HI discs. This is usually attributed to dynamical friction between a misaligned disc and a dark halo. If the dark halo is strongly concentrated, such misalignments are short lived, as is shown also by numerical simulations (Dubinski & Kuijken 1995).
Finally, Navarro (1998) uses the information on a concentration index, derived by fitting a functional form to the observed rotation curves, to argue that dwarf galaxies are too concentrated for a standard CDM = 1 model, but favour instead cosmological models with < 1.
I would like to thank Lia Athanassoula for fruitful discussions and comments on a draft of this paper. I gratefully acknowledge discussions with James Binney, Burkhard Fuchs, John Kormendy, Jerry Sellwood and Peter Vandervoort concerning some of the topics raised in this review.