  It has become clear in the past several years that the cuprates show many
unusual properties, both in the normal and superconducting states, especially
in the underdoped region. In particular, gap-like behavior is observed in
magnetic properties, c-axis conductivity and photoemission, whereas in-plane
transport properties are only slightly affected by the pseudogap. I shall argue
that these experimental evidences must be viewed in the context of the physics
of a doped Mott insulator and that they support the notion of spin charge
separation. I shall review recent theoretical developments, concentrating on
studies based on the t-J model. I shall describe a model based on quasiparticle
excitations, which predicts the doping dependence of T_c and anomalous
energy-gap-to-T_c ratios. Finally, I shall outline how the model may be derived
from a microscopic formulation of the t-J model. After a brief review of the
U(1) formulation, I shall explain some of the difficulties encountered there,
and how a new SU(2) formulation can resolve some of the difficulties.
