Authored by 128 multidisciplinary experts worldwide, the 2025 report of the Lancet Countdown on health and climate change is the ninth—and most comprehensive—assessment of the links between climate change and health. The data in this report reveal that, as the health risks and impacts of climate change break concerning new records, progress is being reversed across key areas, further threatening health and survival. However, the evidence in this report also exposes important opportunities to accelerate action and prevent the most catastrophic impacts of climate change.