  I review methods and techniques to build mass models of disk galaxies from
gas dynamics. I focus on two key steps: (1) the derivation of rotation curves
using 3D emission-line datacubes from HI, CO, and/or H-alpha observations, and
(2) the calculation of the gravitational field from near-infrared images and
emission-line maps, tracing the stellar and gas mass distributions,
respectively. Mass models of nearby galaxies led to the establishment of the
radial acceleration relation (RAR): the observed centripetal acceleration from
rotation curves closely correlates with that predicted from the baryonic
distribution at each galaxy radius, even when dark matter supposedly dominates
the gravitational field. I conclude by discussing the (uncertain) location of
Local Group dwarf spheroidal galaxies on the RAR defined by more massive disk
galaxies.
