Science and secularization have been linked in scholarship and the public imagination. Some suggest that scientific training leads to loss of religion. Yet there is only speculation about the processes by which scientists might become less religious and whether such processes are confined to the west or hold across national contexts. Using original survey data (N = 5,006) of biologists and physicists in India, Italy, and the United States, as well as 215 in-depth interviews, we examine the religious transitions of academic scientists and the factors that they say prompted their religious shifts. We find some support for work suggesting that scientific training is secularizing. Yet we also show that, across national contexts, the nonreligious disproportionately select into scientific careers. Furthermore, we find that scientists tend not to identify science as the primary factor in their own religious transitions. These results challenge long-held assumptions about the relationship between science and secularization.