British nuclear policy faced a major challenge in 1954 when the radiological dangers of the new hydrogen bomb were highlighted by an accident resulting from a U.S. thermonuclear test in the Pacific that underscored how nuclear fallout could travel across national borders. Echoing the response from the United States, the British government downplayed the fallout problem and argued that weapons testing was safe. Some influential scientists rallied behind the government position on fallout and weapons tests, but others disagreed and were regarded within government circles as troublesome dissidents. This article focuses on two of the dissident scientists, Joseph Rotblat and Bertrand Russell, showing how they challenged government policy and sought to make public their view that fallout was dangerous and that weapons testing should stop. Their objections ensured that the fallout debate became a part of public life in Cold War Britain, imbuing the hydrogen bomb and the arms race with new meaning. The article casts new light on the process by which the fallout/testing issue came to be the most publicly controversial area of nuclear weapons policy, serving as a rallying point for scientists beyond the nationstate, at once a national and transnational problem.