Waste—its generation, collection and disposal—is a major global challenge in the 21st century. Cities are responsible for managing municipal waste. Solid waste management is, arguably, the single most important function of cities: waste that is not collected on a timely basis creates a public health hazard, and organic waste, collection vehicles, and most waste disposal methods, with the notable exception of recycling, contribute to greenhouse gas emissions. But the management of waste depends in large part on the global markets for recycled materials.