  Dwarf galaxies are by far the most numerous galaxies in the Universe, showing
properties that are quite different from those of their larger and more
luminous cousins. This review focuses on the physical and chemical properties
of the interstellar medium of those dwarfs that are known to host significant
amounts of gas and dust. The neutral and ionized gas components and the impact
of the dust will be discussed, as well as first indications for the existence
of active nuclei in these sources. Cosmological implications are also
addressed, considering the primordial helium abundance and the similarity of
local Green Pea galaxies with young, sometimes proto-galactic sources in the
early Universe.
