  In this review, the present status of theories for manganites is discussed.
The complex phase diagrams of these materials, with a variety of
spin-charge-orbital ordering tendencies, is addressed using mean-field and
Monte Carlo simulation techniques. The stability of the charge-ordered states,
such as the CE-state at half-doping, appears to originate, in part, in the
topology of the zigzag chains present in those states. In addition, it is
argued that phase separation tendencies are notorious in realistic models for
Mn-oxides. They produce nanoscale clusters of competing phases, either through
an electronic separation tendency or through the influence of disorder on
first-order transitions. These inhomogeneities lead to a ``colossal''
magnetoresistance (CMR) effect, compatible with experiments. This brief review
is based on a more extensive work recently presented [E. Dagotto, T. Hotta, and
A. Moreo, Phys. Rep. {\bf 344}, 1 (2001)]. There, a comprehensive analysis of
the experimental literature can be found. In real manganites, the tendencies
toward inhomogeneous states are notorious in CMR regimes, in excellent
agreement with the theoretical description outlined here.
