What is pollution and why is it important? Why does pollution occur, and is it harmful at all levels? What happens to pollutants in the environment? What are the root causes of pollution? These are among the questions that Chapter 1 will examine. Section I introduces the major impacts that humans exert on Earth's natural systems while also emphasizing our profound dependence on the services provided by those systems. Section II examines why pollution happens, what substances are pollutants, and their sources. Traveling pollutants are described, and the effects they sometimes exert at great distances from their origin. In turn the environment modifies pollutants too, often lessening their harm, especially if levels are not too high. A catastrophic instance of pollution, an explosion at a pesticide plant in Bhopal, India is presented. The opposite extreme, the risk of pollutants in the environment at very low levels is examined too. Section III moves into impoverished parts of the world where pollution sometimes devastates human health. Section IV looks at root causes of pollution, in particular population growth, consumption, and large-scale technology. Finally, Section V comes home to each of us, pointing out that our actions have environmental consequences, sometimes in ways we don't suspect.