This book explains how to use the compartment approach to estimate the distribution of chemical contaminants in environmental media in time and space. This powerful numerical technique enables readers to easily develop the equations that describe complex environmental problems by assembling the equations out of compartmental building blocks. The book offers a user-friendly guide to compartment modeling. It also includes the Generic Environmental Model (GEM) software package, which implements the techniques described. Written to be accessible to beginning environmental modelers, the book is a valuable reference for experienced modelers as well.