  The electronic properties of many transition metal oxide systems require new
ideas concerning the behaviour of electrons in solids for their explanation. A
recent example, subsequent to that of cuprate superconductors, is of rare earth
manganites doped with alkaline earths, namely $Re_{1-x}A_x MnO_3$, which
exhibit colossal magnetoresistance, metal insulator transition and many other
poorly understood phenomena. Here we show that the strong Jahn Teller coupling
between the twofold degenerate ($d_{x^2 -y^2}$ and $d_{3z^2 -r^2}$) $e_g$
orbitals of $Mn$ and lattice modes of vibration (of the oxygen octahedra
surrounding the $Mn$ ions) dynamically reorganizes the former into a set of
states (which we label $\ell$) which are localized with large local lattice
distortion and exponentially small intersite overlap, and another set (labelled
$b$) which form a broad band. This hitherto unsuspected but microscopically
inevitable $coexistence$ of radically different $\ell$ and $b$ states, and
their relative energies and occupation as influenced by doping $x$, temperature
$T$, local Coulomb repulsion $U$ etc., underlies the unique effects seen in
manganites. We present results from strong correlation calculations using the
dynamical mean-field theory which accord with a variety of observations in the
orbital liquid regime (say, for $0.2\stackrel{<}\sim x \stackrel{<}\sim
0.5$).We outline extensions to include intersite $\ell$ coherence and spatial
correlations/long range order.
