The Environmental Protection Agency (EPA or the Agency) is publishing a final rule that identifies which non-hazardous secondary materials, when used as fuels or ingredients in combustion units, are "solid wastes" under the Resource Conservation and Recovery Act (RCRA). This RCRA solid waste definition will determine whether a combustion unit is required to meet the emissions standards for solid waste incineration units issued under section 129 of the Clean Air Act (CAA) or the emissions standards for commercial, industrial, and institutional boilers issued under section 112 of the CAA. In this action, EPA is also finalizing a definition of traditional fuels.