EPA is promulgating amendments to the air emission standards for existing and new large municipal waste combustor (MWC) units. Standards for MWC units were promulgated in 1995 and implemented in 2000. The Clean Air Act (CAA) requires review of these standards every 5 years. The review is to be conducted in accordance with CAA section 129 and section 111 requirements, with standards revised as necessary. For existing MWC units, the goal of this action is to amend the standards to reflect the actual performance levels being achieved by existing MWC units. For new MWC units, the goal of this action is to amend the standards to reflect the performance level achievable by MWC units constructed in the future. Other technical improvements are also being made to the standards for MWC units.