Throughout the previous chapters in this volume, models have been used to illustrate aspects of the dynamics of ecosystems, ranging from energy or nutrient flows through individual species populations, through abstracted food chains to partial models of whole systems. A model is any abstraction or simplification of a system, i.e., it is a simplified version of the real world. Building models of total ecosystems, in contrast to models of component populations or trophic levels, focuses on the ways in which the individual components of the ecosystem are linked to each other, i.e., the ways in which the state variables are linked together and how they interact with the forcing functions.