The Lancet Countdown is an international, multidisciplinary collaboration, dedicated to monitoring the evolving health profile of climate change, and providing an independent assessment of the delivery of commitments made by governments worldwide under the Paris Agreement. The 2019 report presents an annual update of 41 indicators across five key domains: climate change impacts, exposures, and vulnerability; adaptation, planning, and resilience for health; mitigation actions and health co-benefits; economics and finance; and public and political engagement. The report represents the findings and consensus of 35 leading academic institutions and UN agencies from every continent. Each year, the methods and data that underpin the Lancet Countdown’s indicators are further developed and improved, with updates described at each stage of this report. The collaboration draws on the world-class expertise of climate scientists; ecologists; mathematicians; engineers; energy, food, and transport experts; economists; social and political scientists; public health professionals; and doctors, to generate the quality and diversity of data required.