  HI spectral line mapping studies are a unique probe of morphologically
peculiar systems. Not only does HI imaging often reveal peculiarities that are
totally unsuspected at optical wavelengths, but it opens a kinematic window
into the outer dynamics of these systems that is unmatched at any other
wavelength. In this review I attempt to summarize what we have learned from
such studies, and what we may hope to learn from them in the future.
