  We present a summary of recent assessments of the mass distribution in disk
galaxies. Of issue in order to characterize galaxy formation models is to
determine the relative fraction of baryons and dark matter at all radii in
galaxies. For disk galaxies, various measurements of the mass distribution in
galaxies based on vertical kinematics, strong lensing, residuals of scaling
relations, fluid dynamical modeling, bar strength and pattern speed, warps, and
others, have called for either a maximal or sub-maximal contribution of the
baryons in the inner parts of the disk. We propose a global picture whereby all
galaxies are typically baryon-dominated (maximal) at the center and dark-matter
dominated (sub-maximal) in their outskirts. Using as a fiducial radius the peak
of the rotation curve of a pure baryonic exponential disk at R_2.2=2.2 Rd,
where Rd is the disk scale length, the transition from maximal to sub-maximal
baryons occurs within or near R_2.2 for low-mass disk galaxies (with V_tot <
200 km/s) and beyond R_2.2 for more massive systems. The mass fraction's
inverse dependence on circular velocity in disk galaxies is largely insensitive
to the presence of a bar at R_2.2. The mean mass fractions of late- and
early-type galaxies are shown to be different at the same fiducial radius and
circular velocity, pointing to different galaxy formation mechanisms. Feedback,
dynamical friction, size, and IMF variations are key ingredients for
understanding these differences.
