The 21 essays in this two-part book provide conceptual and operational understanding of the nature of models as representations of reality and as tools for description, analysis, interpretation, and forecasting. Topic areas addressed in part 1 (concept) include: the nature of models; the earth as a system; the determination of form; some principles of mathematical modeling; the nature of mathematical models and the problem of consistency; an interactive modeling system as a tool for analyzing complex socio-economic problems; the development of models in urban and regional planning; and a process approach to social goal-setting. Topic areas addressed in part 2 (application) include: global modeling in the 1980s; the politics of model implementation; the validity and usefulness of large economic models; information sources for modeling the national economy; existing and alternative models for the societal use of scientific and technical research; simulating a small nation's international scientific contacts; uses and limitations of models in policy planning and evaluation; micro-operational research; models in science education; a computerized model to understand the brain; quantum theory; the transition to new models in physics; and models of reality. These essays are useful to anyone interested in problem solving, including policy researchers and decision-makers at all levels. (JN)