In few fields of public policy are the use and cost of services so powerfully driven by technological change as they are in medicine. To manage technology, policy-makers have expanded their investment in evaluative research. This paper addresses three underexamined challenges in using evidence: those inherent in the dynamics of technological change itself; those inherent in the analytical enterprise; and the ways in which political factors shape the translation of evidence into policy decisions. The design of institutional arrangements and processes that seek to blend evidence with politics merit closer attention, and existing cross-national arrangements deserve careful study.