Total pollution emitted by U.S. manufacturers declined over the past 30 years, even though manufacturing output increased. This improvement must result from one 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”). In this paper, I first show that most of the decline in pollution from U.S. manufacturing has been the result of changing technology instead of 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 the U.S. manufacturing sector.