In a 6-2 decision, the Supreme Court approved an EPA rule that allows the federal agency to regulate air pollution that crosses from one state into another. The decision is a win for the agency and the Obama administration, and for those states that are downwind of states that produce a lot of pollution. 

The states who fell under the federal solution for the cross-state pollution problem are mainly in the South and the Midwest. Their pollution is carried north and east by the wind. In order to stop those states from polluting their neighbors, the EPA moved to intervene under the Clean Air Act, since the downwind states couldn't regulate industry in other states. Here's a map of the 28 states that fall under the Cross-State Air Pollution Rule (or the transport rules) which the Supreme Court just allowed to take effect: 

Source: EPA 

Under the rule, each upwind state gets a "budget" of the total amount of pollution the state can produce in a year. A large portion of the states, along from representatives of the industries producing that pollution, appealed. Earlier this year, the D.C. Circuit Court struck down the rule, in part because it believed the EPA didn't give the states an opportunity to create their own emissions plans — called a State Implementation Plan, or SIP — to solve the problem. The Supreme Court, however, disagrees, finding that the Clean Air Act requires no such opportunity. In an opinion authored by Justice Ginsburg, the court argues that alternative means of implementing the "good neighbor" provision would, despite the intentions of those opposing the EPA, lead to "costly overregulation" of a complex problem. They conclude: 

We hold that the CAA does not command that States be given a second opportunity to file a SIP after EPA has quantified the State’s interstate pollution obliga­tions. We further conclude that the Good Neighbor Provi­sion does not require EPA to disregard costs and consider exclusively each upwind State’s physically proportionate responsibility for each downwind air quality problem. EPA’s cost-effective allocation of emission reductions among upwind States, we hold, is a permissible, workable, and equitable interpretation of the Good Neighbor 
Provision. 

Justices Scalia and Thomas penned the dissent for the court, and Justice Alito recused himself from the case.