This volume contains the proceedings of a NATO Advanced Study Institute devoted to the study of dynamical correlation functions of the form (I) J~e-lwt< ... > 6ft T- is an equilibrium average. In equation (1) it is useful to regard the product AB as the product of two operators in cases in which A and B refer to different spatial points in a condensed matter sys­ tem and/or in which A and B behave dynamically in a quasiharmonic way. In the second case, one has a two quasiparticle correlation function and CAB;AB(w) gives information about quasiparticle inter­ actions. Condensed matter physics has increasingly turned its attention to correlation functions of this type during the last 15 years, partly because the two point and/or one-particle correlation functions have by now been very thoroughly studied in many cases. The study of four point and/or two quasiparticle correlations has proceeded somewhat independently in several diverse fields of condensed matter physics and it was one purpose of the institute to bring experts from these different fields together to describe the current state of their art to each other and to advanced students.