Summary

Zusammenfassung

One of the main tasks of 21-cm investigations consists in the determination of the density of interstellar Hydrogen gas. This has usually been done with the assumption that the Hydrogen gas forms a stratum of more or less uniform density. If Hydrogen clouds exist, they were believed to be transparent for the 21-cm radiation, and the density gradients were supposed to be small. However, in the last years observations accumulated which indicate this picture to be incorrect. The interstellar material seems to be concentrated in clouds of rather high density and considerable opacity but of small diameter. The kinetic temperatures of these clouds need not to be the same for all clouds, in the contrary, rather large temperature variations are indicated.