Pseudo-differential operators belong to the most powerful tools in the
analysis of partial differential equations. Basic achievements in the
early sixties have initiated a completely new understanding of many old
and important problems in analy- sis and mathematical physics. The
standard calculus of pseudo-differential and Fourier integral operators
may today be considered as classical. The development has been
continuous since the early days of the first essential applications to
ellip- ticity, index theory, parametrices and propagation of
singularities for non-elliptic operators, boundary-value problems, and
spectral theory. The basic ideas of the calculus go back to Giraud,
Calderon, Zygmund, Mikhlin, Agranovich, Dynin, Vishik, Eskin, and
Maslov. Subsequent progress was greatly stimulated by the classical
works of Kohn, Nirenberg and Hormander. In recent years there developed
a new vital interest in the ideas of micro- local analysis in connection
with analogous fields of applications over spaces with singularities,
e.g. conical points, edges, corners, and higher singularities. The index
theory for manifolds with singularities became an enormous challenge for
analysists to invent an adequate concept of ellipticity, based on
corresponding symbolic structures. Note that index theory was another
source of ideas for the later development of the theory of
pseudo-differential operators. Let us mention, in particular, the
fundamental contributions by Gelfand, Atiyah, Singer, and Bott.