This issue explains the concept of externalities (benefits or burdens which accrue to society when there is a difference between the private cost or benefit of an action and the social cost or benefit of that action). These external or social costs of individual actions are often referred to as spillover costs. Three brief teaching units follow the introduction. The grade K-2 unit, "Spillover Detectives," provides activities which require children to identify and investigate examples of both positive and negative externalities. The grade 3-4 unit, "Exploring Externalities," uses decision making, problem solving, and a case study approach to explore the concept of externality. "Consider the Costs," the 5th and 6th grade level unit, provides 3 teaching activities which have students examine negative and positive externalities. (JDH)