  Dwarf galaxies (DGs) are extremely challenging objects in extragalactic
astrophysics. They are expected to originate as the first units in Cold
Dark-Matter cosmology. They are the galaxy type most sensitive to environmental
influences and their division into multiple types with various properties have
invoked the picture of their variant morphological transformations. Detailed
observations reveal characteristics which allow to deduce the evolutionary
paths and to witness how the environment has affected the evolution. Here we
review peculiarities of general morphological DG types and refer to processes
which can deplete gas-rich irregular DGs leading to dwarf ellipticals, while
gas replenishment implies an evolutionary cycling. Finally, as the less
understood DG types the Milky Way satellite dwarf spheroidal galaxies are
discussed in the context of transformation.
