In August 2015, the U.S. Environmental Protection Agency (EPA) finalized the Clean Power Plan for existing power plants. Under the rule, states can implement a mass-based or rate-based compliance plan to reduce greenhouse gas emissions from the power sector. States choosing a mass-based approach must also decide how to allocate emission allowances. This fact sheet provides an overview of how allowances could be distributed under a mass-based approach and the policy objectives achieved by their distribution.