Scientists are often reluctant to speak in public on contested issues, for fear that this will “politicize” their science and have a negative impact on their credibility. In this talk, I examine these concerns, by exploring historical examples of scientists who have spoken up on scientific issues of broad importance, including nuclear weaponry, ozone depletion and climate change. These examples suggest that, while becoming a public figure does entail risks, there is little basis in history for the concern that it undermines the credibility of one’s scientific work. Moreover, these examples suggest that society needs scientists to speak up to alert society to challenges that, without science, we would not understand and might not even recognize. Yet they also do point to certain limits to what scientists can and should do as public figures.