Historians of science have participated actively in debates over American science policy in the post-World War II period in a variety of ways, but their impact has been more to elucidate general concepts than to effect specific policy changes. Personal experiences, in the case of the debate over global warming, have demonstrated both the value and the limits of such involvement for the making of public policy. To be effective, historians of science need to strive for clarity in public expression, to accept the importance of engaging with the public at all levels and through diverse media, and, above all, to recognize that the nature of such debates will make normal scholarly nuance hard to achieve. Moreover, in the current political climate, historians may be surprised to find themselves defending sciences, when the usual stance of historians is to be critical.