The authors present a case study examining the potential for policies to be “evidence-based.” To what extent is it possible to say that a decision to implement a complex social intervention is warranted on the basis of available empirical data? The case chosen is whether there is sufficient evidence to justify banning smoking in cars carrying children. The numerous assumptions underpinning such legislation are elicited, the weight and validity of evidence for each is appraised, and a mixed picture emerges. Certain propositions seem well supported; others are not yet proven and possibly unknowable. The authors argue that this is the standard predicament of evidence-based policy. Evidence does not come in finite chunks offering certainty and security to policy decisions. Rather, evidence-based policy is an accumulative process in which the data pursue but never quite capture unfolding policy problems. The whole point is the steady conversion of “unknowns” to “knowns.”