small fraction of the light. Plausible variations in the assumed initial mass function (IMF) can therefore change the $M/L$ without affecting significantly the characteristics of the observed light (e.g., Bell & de Jong 2001). Accurate measurements of galaxy $M/L$ values therefore generally rely on dynamical properties. The speed at which stars or gas move probes directly the gravitational force to which they are subjected, and therefore the mass of the system.

The quality of the kinematical data for galaxies in the local Universe, as well as the methods by which their dynamics can be modeled, have steadily increased in sophistication over the years. Large samples of reliable $M/L$ measurements are now available from many studies, in particular for early-type galaxies (e.g., van der Marel 1991; Magorrian [[et al. ]{}]{}1998; Gebhardt [[et al. ]{}]{}2003; Cappellari [[et al. ]{}]{}2006a). For these galaxies it is well established that the $M/L$ inside an effective radius is relatively constant, and contains only a small contribution from the dark halo (e.g., Kronawitter [[et al. ]{}]{}2000). Therefore, the inferred $M/L$ values are primarily driven by the characteristics of the stellar population. This situation differs considerably from the case for spiral galaxies. While mass profiles have been derived 