This book originated from a course given at the Univcrsidad Aut6noma of Madrid in the Spring of
1994 and in the Universidad Complutense of Madrid in 1995. The goal of these courses is to give
the non-specialist an introduction to some old and new ideas in the field of strongly
correlated systems in particular the problems posed by the high-1~ superconducting materials.
As theoretical physicists our starting viewpoint to address the problem of strongly correlat
ed ferlnion systems and related issues of modern condensed matter physics ·is the
renormalization group approach applied both to quantU111 field theory and statistical physics.
In recent years this has become not only a powerful tool for retrieving the essential physics
of interacting systems but also a link between theoretical physics and modern condensed matter
physics. Furthermore once we have this common background for dealing with apparently different
prob lems we discuss more specific topics and even phenomenological aspects of the field. In
doing so we have tried to make the exposition clear and simple with out entering into
technical details but focusing ill the fundamental physics of the phenomena under study.
Therefore ve expect that our experience ll1ay have some value to other people entering this
fascinating field. We have divided these notes into three parts and each part into chapters
which correspond roughly to one or two lectures. Part I Chaps. 1-2 (A. H. V.
