Climate change threatens to undermine the last 50 years of gains for public health, intensifying heatwaves and extreme weather events, worsening flood and drought, altering the spread of infectious diseases, and exacerbating poverty and mental ill-health. Crucially, the response to climate change brings immense benefits for human health, with cleaner air, healthier diets, and more liveable cities. The Lancet Countdown: Tracking Progress on Health and Climate Change exists to monitor this transition from threat to opportunity. We are a collaboration of over 120 leading experts from 35 universities and UN agencies across the globe, bringing together climate scientists, engineers, energy, economists, political scientists, public health professionals, and doctors. Each year our findings are published annually in medical journal The Lancet ahead of the UN climate change negotiations. Our data makes clear how climate change is affecting our health, the consequences of delayed action and the health benefits of a robust response. The Lancet Countdown works to ensure that health is at the centre of how governments understand and respond to climate change. This ranges from ensuring policymakers have access to high-quality evidence-based guidance, through to providing the health profession with the tools they need to improve public health.