Dark Matter in Disc Galaxies

Recent work on the mass distribution in spiral galaxies, using mainly HI observations, is reviewed. The principal problem is still to determine to what extent the dark matter is important in the inner parts of a galaxy, or in other words, how dominant is the self-gravitation of the disc. Studies of the shapes of rotation curves show that in detail there is sufficient individuality in spiral galaxies to prohibit the construction of ``Universal Rotation Curves''. A detailed account is given of the method of Athanassoula et al. (1987), where swing amplifier criteria are applied to set a range in the mass-to-light ratio of the disc. To restrict this range further, other methods might be useful. For a number of bright spirals the rotation curve drops just outside the optical image, but this feature by itself cannot constrain unambiguously the mass models. The use of velocity dispersions seems a promising way, though the observational problems are hard. Within the uncertainties, discs can be close to ``maximum'', even though a range of values cannot be excluded.