The observational evidence on the discrepancy between the mass distribution in galaxies derived from HI rotation curves and that derived from the distribution of light is reviewed. In the outer parts the discrepancy is such that in some galaxies there is at least three times as much dark matter as luminous matter. This is a direct consequence of the nearly constant circular velocity far beyond the edge of the visible part of the galaxy, as derived from the motion of HI. The discrepancy is clearly present already near the edge of the visible disk (R25). In the inner regions, i.e. inside approximately 2.5 disk scale-lengths, no dark matter is required, but its presence can not be ruled out. There is no evidence for a dependence on galaxy luminosity or morphological type. These results suggest a strong coupling between luminous matter and dark matter within individual galaxies, and among galaxies as well. Finally attention is drawn to the large-scale asymmetries in the outer parts of galaxies and to possible implications for the vertical distribution of dark matter.