“Climate change is the greatest threat to global health in the 21st century”—that is the World Health Organization’s web-based dispatch for immediate action. The WHO’s director-general adds, “The evidence is overwhelming: climate change endangers human health. Solutions exist and we need to act decisively to change this trajectory.” Expected consequences include the spread of disease, drought, and forced migration due to expected weather events and trends. Despite the gravity and relevance to health of these phenomena, roles of health care professionals in responding to and roles of organizations and governments in implementing policies and action plans have neither been clarified nor, by some, considered. This month's issue will explore clinical and ethical questions about domestic and global health consequences of climate change and the nature and scope of the obligations that clinicians, organizations, and governments have to address them.