Given the growing support for populist actors and the expected continued relevance of COVID-related issues for pandemic policy, this project investigates the interrelationship between populism and COVID-19 related conspiracy theories in four countries – Switzerland, Austria, Italy and France – where populist parties have been long established and have different degrees of relevance in the political system. To allow for a joint supply-side and a demand-side analysis, this project adopts a mixed-method approach combining quantitative and qualitative text analysis of populist actors’ communication, with a dedicated survey and three survey experiments (vignette, framing, conjoint) on attitudes related to COVID-related conspiracy theories fielded in all four countries. The analysis explores the relationship between populism and conspiracist discourses and its impact on policy and seeks to account for the variation between the cases.