 This study examined the effectiveness of health in all policies, high AP, approaches in improving population health and reducing health inequalities. It found that successful high AP initiatives require both internal and external legitimacy and credibility, which are generated through structural and relational components. These include adequate resources, meaningful community engagement, and leadership that champions collaborations and identifies win-win strategies. Additionally, external factors such as economic shocks and short political cycles can reduce collaborative power. This article was authored by Elizabeth Such, Catherine Smith, Helen Buckley Woods, and others.