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.
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