Total pollution emitted by U.S. manufacturers declined over the past 30 years by about 60 percent, even though real manufacturing output increased 70 percent. This improvement must result from a combination of two trends: (1) changes in production or abatement processes ("technology"); or (2) changes in the mix of goods manufactured in the United States, which itself may result from increased net imports of pollution-intensive goods ("international trade"). I first show that most of the decline in pollution from U.S. manufacturing has been the result of changing technology, rather than changes in the mix of goods produced, although the pace of that technology change has slowed over time. Second, I present evidence that increases in net imports of pollution-intensive goods are too small to explain more than about half of the pollution reductions from the changing mix of goods produced in the United States. Together, these two findings demonstrate that shifting polluting industries overseas has played at most a minor role in the cleanup of U.S. manufacturing.