This review summarizes information obtained from studies of dark matter, of optical interstellar absorption lines, and of the 21-cm line observed in absorption and in emission.Great improvements in the velocity resolution of interstellar lines (Section 4) are making possible a detailed analysis of their component structure, and determination of the internal motions in the clouds. The 21-cm absorption spectra of strong radio sources (Section 5) have yielded statistics of the optical depths and the internal velocity dispersions of absorbing clouds. Studies of the 21-cm line in emission (Section 6) supply a wealth of data on internal motions in the clouds. Comparisons of Gaussian component parameters at neighbouring positions allow delineation of cloud boundaries on the sky, but estimates of size and mass depend on (usually unknown) distance. The Green Bank telescope has revealed the existence of ‘cloudlets’ containing a few solar masses within a volume of about 5 pc diameter.Structural details in the interstellar medium show a wide variety of size, density and mass. Recent studies tend to emphasize the smaller and thinner structures. The external cloud motions are best approximated by an exponential velocity distribution, with a root-mean-square velocity of 7 km/sec in one coordinate. Internal motions within the clouds probably have Gaussian distributions, with velocity dispersions ranging from o·6 to possibly 7 km/sec; part of these may be due to non-thermal mass motions.