The paper reviews the nature of the problems associated with toxic metals in combustion processes, and describes where these problems occur and how they are addressed through current and proposed regulations. Although emphasis in the paper is on problems associated with metals from incineration processes, conventional fossil fuel combustion is also considered, insofar as it pertains to mechanisms governing the fate of metals during combustion in general. The paper examines the release of metals into the vapor phase, with the particle dynamics of a nucleating, condensing, and coagulating aerosol that may be subsequently formed, and with the reactive scavenging of metals by sorbents.