  With the advent of the next generation wide-field cameras it became possible
to survey in an unbiased mode galaxies spanning a variety of local densities,
from the core of rich clusters, to compact and loose groups, down to filaments
and voids. The sensitivity reached by these instruments allowed to extend the
observation to dwarf galaxies, the most "fragile" objects in the universe. At
the same time models and simulations have been tailored to quantify the
different effects of the environment on the evolution of galaxies. Simulations,
models, and observations consistently indicate that star-forming dwarf galaxies
entering high-density environments for the first time can be rapidly stripped
from their interstellar medium. The lack of gas quenches the activity of star
formation, producing on timescales of ${\sim}$1~Gyr quiescent galaxies with
spectro-photometric, chemical, structural, and kinematical properties similar
to those observed in dwarf early-type galaxies inhabiting rich clusters and
loose groups. Simulations and observations consistently identify ram pressure
stripping as the major effect responsible for the quenching of the
star-formation activity in rich clusters. Gravitational interactions (galaxy
harassment) can also be important in groups or in clusters whenever galaxies
have been members since early epochs. The observation of clusters at different
redshifts combined with the present high infalling rate of galaxies onto
clusters indicate that the quenching of the star-formation activity in dwarf
systems and the formation of the faint end of the red sequence is a very recent
phenomenon.
