This book is not an introductory text on environmental fate and transport modeling. There are many excellent references on that general topic elsewhere (Thomann, 1972; Thomann and Mueller, 1987; Mackay, 1991; Schnoor, 1996; Chapra, 1997; Ramaswami et al., 2005; Dunnivant and Anders, 2006). This book is an introduction to a speci•c numerical modeling technique-the “compartment” approach-as well as a user’s guide to the GEM.