  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.
