'Planetary health' is a new field of research, education and practice focusing on the relationship between global environmental change and human health. This includes climate change, but also biodiversity loss, environmental pollution, and other large-scale changes in the natural environment that may affect human health. This article provides an overview of the extent to which scientific knowledge is available about these health risks. Both the scientific literature and expert opinion indicate that global environmental changes may have potentially disastrous consequences for human health worldwide. Countermeasures are therefore indicated, both in the form of mitigation (combating global environmental change) and in the form of adaptation (e.g., to limit the health consequences). The health care sector has an important responsibility, also because of its own contribution to global environmental change, and both health care practices and medical education will have to change in response to the health risks of global environmental change.