The standard way to construct a mass model for a spiral galaxy is to make composite disc/halo models assuming a ``maximum disc'' solution, or to adopt a ``best fit''. In such models, the data from surface photometry are used, assuming constant mass-to-light ratios, to calculate the expected rotation curve for the visible components, bulge and disc. From the observed HI gas density, and a suitable factor to include helium, a rotation curve is calculated also, and quadratically added to the first one. The resulting curve is then compared with the observed rotation curve, and an additional dark halo component is introduced when necessary.
For extended HI rotation curves, such an analysis has been done by several authors, e.g. Begeman et al. (1991). The constancy of the mass-to-light with radius within each component is usually justified by the absence of colour gradients. Indeed, data of De Jong (1996) shows that colour gradients are small, and if present, they are in the sense that discs become bluer outwards. In that case the use of near infrared data is preferred, since it accounts better for the contribution to the mass of the old stellar disc. From De Jong's (1996) data, it follows that the ratio of the blue (B-band) and near infrared (K'-band) scalelengths is about 1.15 for 85% of his sample. However, from Verheijen's studies on the spirals in the Ursa Major cluster (Verheijen 1997), a much larger spread in the ratio of scalelengths is seen (cf. Tully et al. 1996).
The primary trend in these models is that the importance of dark matter increases with decreasing luminosity : small galaxies seem to be more dark matter dominated than large galaxies (cf. Broeils 1992). Another trend is that the rotation curve of the gas component has more or less the same shape as the halo rotation curve : if all matter were is a disc, the ratio of gas mass to total mass becomes constant in the outer parts (cf. Bosma 1978, 1981b; Carignan et al. 1990).
However, such results, based as they are on the assumption of maximum disc, may not be correct. In this review, I will comment on the recent work on rotation curves and mass models of spiral galaxies, and in particular address part of the question whether the maximum disc hypothesis is justified. For another discussion on this last topic, using arguments principally concerning barred galaxies, see Sellwood (this volume).