The precautionary principle recommends, in the face of pressing but unquantifiable threats, that decision makers should not wait indefinitely for the backing of evidence. As such, it represents another potential challenge to the turbulent fortunes of evidence-based policy. Through four contrasting case studies, this paper examines this challenge and formulates a preliminary model of the role of the precautionary principle in evidence-based policy. We conclude that although precaution may be a barrier to evidence-based policy making, this is not always so and depending on the contextual factors, it can also be enabling, encouraging policy makers to engage with the evidence.