The report is an overview of waste co-firing and auxiliary fuel fired technology and identifies the extent to which co-firing and auxiliary fuel firing are practiced. Waste co-firing is defined as the combustion of wastes (e.g., sewage sludge, medical waste, wood waste, and agricultural waste) in a unit designed to burn municipal solid waste (MSW) or refuse derived fuel (RDF) as a major fraction of total fuel input. Auxiliary fuel firing is defined as firing coal, fuel oil, or natural gas in a municipal waste combustor under conditions when waste feed quantities are interrupted. This is a fairly common practice for dedicated RDF boilers, and there may be additional mass burn MWCs that meet the definition of auxiliary fuel firing. The report describes technologies used by facilities that meet these definitions, characterizes the population, and discusses design and operating practices and available emissions data from each facility. The report concludes with a discussion of recommended good combustion practices for waste co-firing combustors and auxiliary fuel fired MWCs.