  The thermodynamic and kinetic anomalies of supercooled liquids are analyzed
from the perspective of energy landscapes. A mean field model, a generalized
random energy model of liquids is developed, which exhibits a dynamical
transition of the onset of slow dynamics at T_0, alteration of the nature of
motion from the saddle-to-saddle to minimum-to-minimum motion at T_c, and an
ideal glass transition at T_k. If the energy spectrum of the configurations has
a low energy tail, the model also allows a thermodynamic liquid-liquid
transition at T_l. The liquid-liquid transition of the model is correlated to
the kinetic fragile-strong transition accompanied by the anomalous slowing down
of motion. Fragility of the system is classified in terms of features of the
energy landscape such as ruggedness of the potential energy surface, size of
the cooperative motion invoked in a transition from one configuration to
another, and energy needed to deform the local structure in the cooperative
motion. A simple relation is found between diffusion constant, D and the saddle
index of the potential energy surface, f, as $D \propto f^{a}$, where a depends
on the size of the cooperative motion.
