David Nyhan of the Boston Globe described how air pollution is linked to global change, “Wind, rain and radioactivity do not stop at the border for passport control, but go where they will. Pollution? Coming soon to a place near you … We're all down-winders now.” You have already encountered traveling pollutants in this text: radioactive substances from the Chernobyl explosion, persistent organic pollutants traveling to the Arctic, mammoth dust storms from Africa and Asia reaching North America, and smoke from giant fires. This chapter focuses on another major category of traveling pollutants: acidic substances and their precursors. It asks what happens after acids deposit from air onto Earth and water, as happens in many regions around the world. In this chapter, Section I identifies the major pollutants responsible for acid deposition, and describes how they are formed. It overviews a half-a-billion-dollar study carried out in the United States to better understand acid deposition. Section II examines the adverse effects of acid deposition on water and aquatic life, and on forests and their soils. Section III looks at emission sources of acid-forming pollutants, and how to reduce emissions. Section IV moves on to international issues around the subject of acid deposition.