  Theoretical studies on charge ordering phenomena in quarter-filled molecular
(organic) conductors are reviewed. Extended Hubbard models including not only
the on-site but also the inter-site Coulomb repulsion are constructed in a
straightforward way from the crystal structures, which serve for individual
study on each material as well as for their systematic understandings. In
general the inter-site Coulomb interaction stabilizes Wigner crystal-type
charge ordered states, where the charge localizes in an arranged manner
avoiding each other, and can drive the system insulating. The variety in the
lattice structures, represented by anisotropic networks in not only the
electron hopping but also in the inter-site Coulomb repulsion, brings about
diverse problems in low-dimensional strongly correlated systems. Competitions
and/or co-existences between the charge ordered state and other states are
discussed, such as metal, superconductor, and the dimer-type Mott insulating
state which is another typical insulating state in molecular conductors.
Interplay with magnetism, e.g., antiferromagnetic state and spin gapped state
for example due to the spin-Peierls transition, is considered as well. Distinct
situations are pointed out: influences of the coupling to the lattice degree of
freedom and effects of geometrical frustration which exists in many molecular
crystals. Some related topics, such as charge order in transition metal oxides
and its role in new molecular conductors, are briefly remarked.
