There are two general approaches to environmental regulation: command-and-control regulation and market-based instruments. Command-and-control regulation is the most common approach to address environmental problems around the world. It works by setting standards and directly regulating the activities of firms and individuals. The Clean Air Act in the United States is an example of a successful command-and-control environmental policy. In the case of climate change, standards could be applied to energy using devices (cars, appliances, homes), the carbon content of fuels, and the mix of energy sources used to generate electricity.