Jon Agar, in his sprawling history of recent science and technology, Science in the 20th Century and Beyond, identifies secret and classified research as one of the major underexplored dimensions of history of twentieth-century science. If measured in person-hours, rather than publications, secret science most certainly rivals all other major sectors of research in output. But, by design, it is difficult to document. Decades after the fact, classification remains sticky. Few historians have security clearances and even when they do, they are restricted in what they can discuss in print. This led Alex Wellerstein to forsake the more comprehensive but unshareable knowledge that a clearance would provide in developing his account of the way that the Manhattan Project has shaped nuclear secrecy in the United States, Restricted Data. Secret science represents not only a large lacuna in the historiography of twentieth-century science, but a distinctively awkward one.

Two articles in this issue respond to this challenge in different ways. Girardo Ienna and Simone Turchetti examine the wages of secrecy by focusing on the scientists engaged in secret research. The JASONs are among the most famous expressions of the secretive scientific security state during the Cold War. This group of elite scientific advisors on US nuclear policy held down day jobs, often in traditional university departments, where they also published in the open literature. Ienna and Turchetti examine the global stakes of their moonlighting, showing how the JASONs’ fame influenced their reception in a Europe that was growing increasingly skeptical of the United States, and the guardians of its secret research, in the context of the Vietnam War.

Also focusing on the European context, Machiel Kleemans examines declassification, which both brought previously secret research into the open, and functioned as a tool to advance geopolitical aims. The physical and psychological power of nuclear weapons meant that, in the late 1940s, the United States used its classification powers liberally on anything with a nuclear patina. Reactor research, work with radioactive tracers, and other so-called peaceful uses of nuclear science were apt to be caught in this wide net. When, whether, and how they were extracted from it often depended on complex political considerations, and so studying those decisions sheds light on the politics of secrecy.

Science, to the extent that it can be said to have a self-image, does not consider itself secretive. In the ideal image of science, it is animated by openness; it advances through the free exchange of ideas and information and its progress is inhibited to the extent that the flow of those ideas and of that information is curtailed. This is the image of science opponents of classification regimes invoked through the middle of the twentieth century. But despite these protestations, secret science proliferated, and continues to proliferate, to extents we still do not fully understand. These two articles showcase some strategies for encouraging a greater appreciation of its importance.