Continuum observations of NGC 6946 strongly suggest the presence of an exponential non-thermal disk component. When applied to galactic 408 MHz observations, the scale length is found to be between 5 and 6 kpc. Continuing the parallel with NGC 6946, a total luminosity of 2 × 1010 L⊙ is derived. Observations of the edge-on galaxies NGC 891 and 4631 show the existence of flattened halos with a steep spectrum around these galaxies. This is consistent with the constraints that are known for the galactic radio halo. On the basis of the radial distribution of HI in the Galaxy and in M81 three regimes can be defined: a central one which is hyperdeficient in hydrogen; a middle one which is deficient in HI, but not necessarily in total hydrogen; and an outer one which is neither deficient in HI, nor in H. Density wave theory, combined with gas flow dynamics appear a powerful tool in interpreting the kinematical data. No rotation curve is yet known to become Keplerian in the outer parts. Many external galaxies are warped, like our own; one of them, NGC 5907, does not have a visible companion.