Controversy over vaccine safety has achieved high visibility over the past decade. At the same time, however, levels of coverage for routinely recommended childhood vaccines in the United States are at their highest ever. We examine this apparent paradox. We consider the ways in which concerns over vaccine safety have emerged and diffused through the popular media, legislative hearings, and Internet-based activism. As a case study, we review the controversy over the alleged connection between autism and the measles-mumps-rubella (MMR) vaccine and consider why it had a dramatic effect on the vaccine's acceptance in Great Britain but virtually none in the United States.