This volume gathers the invited talks of the XIII International Work shop on Condensed Matter Theories which took place in Campos do Jordao near Sao Paulo, Brazil, August 6-12, 1989. It contains contributions in a wide variety of fields including neutral quantum and classical fluids, electronic systems, composite materials, plasmas, atoms, molecules and nuclei, and as this year's workshop reflected the natural preoccupation in materials science with its spectacular prospect for mankind, room tempera ture super-conductivity. All topics are treated from a common viewpoint: that of many-body...