Health policy makers and clinicians often face similar decision-making challenges. The issues are turbulent, characterised by high risk and complexity, often involve value conflicts and occur in settings of rapid change. Policy makers' decisions are under increasing scrutiny for their use of evidence, with many health policies reflecting political influence rather than rigorous analysis. The evidence-based policy movement offers a range of accounts for this. We argue that advocacy in three critical areas helps explain when evidence is used in the policy making process and then contrast the impacts of advocacy for evidence use in two nutrition policy cases.