Dwarf galaxies (DGs) are more numerous than large galaxies. Most dwarfs in clusters are dEs, but in the field they belong mostly to late types. The importance of late-type DGs in the context of the intergalactic medium (IGM) lies in the fact that (at least some) are probably ``young'' galaxies forming stars for the first time. Many DGs have significant amounts of interstellar gas, they may form out of intergalactic gas clouds, and matter ejected from such galaxies as a result of star formation processes may enrich the IGM with metals. The physical mechanism responsible for triggering the star formation process has not yet been identified. The extended halos of DGs may provide (at least some of) the QSO absorption line systems. Recent observations also show that the LSB dwarf galaxies may be good tests of MOND.

To be published in the proceedings of the XVI Cracow Summer School of Cosmology, 9 pages, no figures %in the University of Lodz Folia