  We present Very Large Array ({\sc vla}) and Westerbork Synthesis Radio
Telescope ({\sc wsrt}) 21-cm H{\sc i} observations of 19 late-type low surface
brightness (LSB) galaxies. Our main findings are that these galaxies, as well
as having low surface brightnesses, have low H{\sc i} surface densities, about
a factor of $\sim 3$ lower than in normal late-type galaxies. We show that LSB
galaxies in some respects resemble the outer parts of late-type normal
galaxies, but may be less evolved. LSB galaxies are more gas-rich than their
high surface brightness counterparts. The rotation curves of LSB galaxies rise
more slowly than those of HSB galaxies of the same luminosity, with amplitudes
between 50 and 120~km~s$^{-1}$, and are often still increasing at the outermost
measured point. The shape of the rotation curves suggests that LSB galaxies
have low matter surface densities. We use the average total mass surface
density of a galaxy as a measure for the evolutionary state, and show that LSB
galaxies are among the least compact, least evolved galaxies. We show that both
$M_{\rm HI}/L_B$ and $M_{\rm dyn}/L_B$ depend strongly on central surface
brightness, consistent with the surface brightness--mass-to-light ratio
relation required by the Tully-Fisher relation. LSB galaxies are therefore
slowly evolving galaxies, and may well be low surface density systems in all
respects.
