PAHs are primarily emitted by anthropogenic activities, and although some natural and secondary sources of these contaminants are discussed, the focus of this chapter is the major primary sources to the atmosphere. We examine the thermal reactions involved in the release of PAHs, and include details of some important non-combustion sources. Section 4.2.5 looks at the study of substituted derivatives of the 'parent' PAHs as a tool for the identification of sources. Source apportionment techniques are important for some of the source inventory and budgetary studies discussed in Sect. 3, which examines the concepts and uncertainties inherent in studies of this kind, and assesses the importance of historical monitoring techniques in providing information on the trends in emissions of PAHs. The UK is taken as a budgetary and inventory case-study, and other national and regional approaches to source inventories and budget studies are also examined. Although primary sources of PAHs are thought to outweigh secondary sources, section four assesses the importance of environmental recycling as a secondary source of PAHs, and also looks at the balance between environmental burden and primary sources of PAHs.