HI galaxy dynamics and dark matter

I review the mass distribution of galaxies as probed by H I kinematics for ellipticals, spirals and dwarf galaxies. The most detailed and strongest evidence for dark matter in galaxies has always been provided by extended H I rotation curves and, especially, amongst all the galaxy types, the rotation curves of late-type spirals and dwarf irregular galaxies. The following issues will be discussed: i) the 'maximum-disk' issue ' i.e. what is the relative mass contributions of the stellar disk and the dark matter halo? ii) the viability of Cold Dark Matter (CDM) models-can the discrepancy between CDM rotation curves and observed ones be explained by beam-smearing? iii) the viability of MOND (Modified Newtonian Dynamics). iv) the coupling of H i and dark matter: can one use a scaled version of the observed H i surface density to describe the dark matter surface density? I will also describe ongoing work on extending rotation curves to very large galactocentric radii using background QSOs.