  A galaxy's mean metallicity is usually closely correlated with its luminosity
and mass. Consequently the most metal-poor galaxies in the local universe are
dwarf galaxies. Blue compact dwarfs and tidal dwarfs tend to deviate from the
metallicity-luminosity relation by being too metal-poor or too metal-rich for
their luminosity, respectively. A less pronounced offset separates dwarf
spheroidal (dSph) and dwarf irregular galaxies, making the former too
metal-rich for their luminosity, which indicates different formation conditions
for these two types of dwarfs. While environment (photo-evaporation through
local re-ionization by massive galaxies, tidal and ram pressure stripping)
govern the observed morphology-distance relation, intrinsic properties (in
particular total mass) play a decisive role in dwarf galaxy evolution with
respect to the time and duration of star formation and the amount of
enrichment. The metallicity distribution functions of nearby dwarfs can be
understood taking pre-enrichment, gas infall, and winds into account. Many
dwarfs show evidence for inhomogeneous, localized enrichment. Ultra-faint
dSphs, which may have formed their metal-poor stars at high redshift via H2
cooling, show an overabundance of metal-deficient stars as compared to the
(inner) Galactic halo, but may, along with classical dSphs, have contributed
significantly to the build-up of the outer halo. The abundance ratios measured
in the irregular Large Magellanic Cloud are consistent with the postulated
early accretion of irregulars to form the inner Galactic halo.
