More than any other institution or organization in the world, the United Nations has been at the forefront of efforts to address climate change. The UN has been a leader in assessing the state of the science through the reports of the Intergovernmental Panel on Climate Change. It has brought countries together through the UN Framework Convention on Climate Change (UNFCCC), and is currently working to negotiate a universal and meaningful climate agreement. And it is working with countries on strategies, policies and projects to promote a low-carbon economy. Climate change is the defining issue of our time. It is the ultimate test for the global community to unite and meet a challenge that respects no borders. The effects of climate change are already widespread, costly and consequential. But there are many things we can do now, with existing technologies, to address it. This booklet puts the climate change activities of the UN system — which includes about 40 specialized agencies, funds, programmes and other bodies — into the broader context of sustainable development. It presents the story of how the UN is working with all levels of government, business and civil society as an effective partner. The range of actions is vast: it includes efforts to improve local weather forecasts; diminish deadly air pollution; minimize risk from disasters; manage and conserve forests, reduce deforestation and assist forest communities; cut emissions from transport on air, land and sea; and make urban areas more sustainable, energy efficient and liveable. The UN System is committed to advancing solutions that will build prosperous economies and more resilient communities while addressing climate change. By acting on climate change we can significantly advance the sustainable development agenda.