This volume consists of two articles of particular interest to researchers in the field of statistical mechanics. Its appeal is, however, not limited to this group. Both articles are written in a style that makes them accessible to graduate students and to scientists who are interested in but not specialists in this field. The first article is based on the premise that the best way to understand the qualitative properties that characterize many-body (i.e. macroscopic) systems is to study ``a number of the more significant model systems which, at least in principle are susceptible of complete analysis.'' The second article deals exclusively with nonequilibrium phenomena. It reviews the theory of fluctuations in open systems to which the authors have made important contributions.