  Irregular (Irr) galaxies are gas-rich objects with recent or ongoing star
formation. In absence of spiral density waves, star formation occurs largely
stochastically. The scattered star-forming regions tend to be long-lived and
migrate slowly. Older populations have a spatially more extended and regular
distribution. In fast-rotating Irrs high star formation rates with stronger
concentration toward the galaxies' center are observed, and cluster formation
is facilitated. In slowly or nonrotating dwarf Irrs (dIrrs) star formation
occurs more quiescently, and the formation of OB associations is common. Irrs
and dIrrs tend to experience continuous star formation with amplitude
variations and to continue to form stars for another Hubble time. Irrs exhibit
lower effective yields than spirals, and [alpha/Fe] ratios below the solar
value. This may be indicative of fewer Type II SNe and lower astration rates in
their past. Alternatively, many metals may be lost from the shallow galaxy
potential wells due to selective winds. The differences in the
metallicity-luminosity relation between dIrrs and dwarf spheroidals (which,
despite their lower masses, tend to have too high a metallicity for their
luminosity as compared to dIrrs) further supports the idea of slow astration
and slow enrichment in dIrrs. The current data on age-metallicity relations are
still too sparse to distinguish between infall, leaky-box, and closed-box
models. The preferred location of dIrrs in the outer parts of galaxy groups and
clusters and in the field as well as the positive correlation between gas
content and distance from massive galaxies indicate that these dIrrs probably
have not yet experienced significant interactions or galaxy harassment.
(Abridged)
