  We review the recent progress in the theory of inhomogeneous superconductors.
It was shown that Gor'kov's self-consistency equation needs a pairing
constraint derived from the Anomalous Green's function. The Bogoliubov-de
Gennes equations also need a pairing constraint in order to obtain a correct
vacuum state by the corresponding unitary transformation. This new study opens
up a reinvestigation of inhomogeneous superconductors. We discuss (i) problems
of the conventional Green's function theory, (ii) reinvestigation of impure
superconductors, and (iii) impurity doping effect in high $T_{c}$
superconductors. It is also pointed out that a new formalism is required to
tackle the macroscopically or mesoscopically inhomogeneous systems such as the
junction and the vortex problems.
