I aim to clarify the relationship between the success of a theory and the truth of that theory. This has been a central issue in the debates between realists and anti-realists. Realists assume that success is a reliable indicator of truth, but the details about the respects in which success is a reliable indicator or test of truth have been largely left to our intuitions. Lewis (Synthese 129:371–380, 2001) provides a clear proposal of how success and truth might be connected, comparing a test of success of our theories to medical tests with low rates of false positives and false negatives. But, contrary to what Lewis claims, I argue that it is not enough for the realist to undercut the claim that success is not a reliable indicator of truth. Rather, the realist must show that our current best theories are likely true. Further, I argue that tests in science are unlike medical tests in a number of important ways.