The scientific consensus is that climate change affects health through changing weather patterns (such as more intense and frequent extreme events) and indirectly through changes in water, air, food quality and quantity, ecosystems, agriculture, livelihoods and infrastructure. The effects will be unevenly distributed, and those at greatest risk include people who are poor, very young, elderly, and/or ill. Climate change can also pose a threat to health security. Failure to respond could be very costly in terms of disease health care expenditure and lost productivity. As long as climate change is not too rapid or strong, strengthening health systems can control many of the health effects. This may include strengthening preparedness, public health services and health security, advocating action in other sectors to benefit health, better informing citizens and leading by example. Health systems need to strengthen their capacity to assess potential climate-related health effects, to review their capacities to cope, and develop and implement adaptation and mitigation strategies, and to strengthen a range of key areas of work - from disease surveillance and control to disaster risk reduction - that are essential for rapid detection of and action against climate-related risks. This publication intends to stimulate debate and support an active response by providing up-to-date information on the health effects of climate change as well as practical guidance on specific actions that decision-makers at different levels in health and other sectors can take now.