Although there are many potential treatment technologies, none is as universally applicable as incineration to the treatment of the types of solid waste governed by the different Federal laws in the United States. However, there is an increasing concern over the emission of unknown combustion by-products from incineration sources. This Paper is to address the issue of combustion by-products (CBPs) (also generally known as the products of incomplete combustion (PICs) from the following major solid waste thermal treatment activities: (1) hazardous waste incineration; (2) municipal waste incineration; (3) medical waste incineration; (4) Superfund waste incineration; (5) toxic substances incineration; and (6) sludge wast incineration. To address the CBP issue from its roots, this Paper will discuss its regulatory framework that exists in the United States.