A comprehensive review of available PCDD/F time trend data is presented. This focuses on industrialized countries, drawing heavily on those countries that have been actively involved in PCDD/F monitoring and research, notably Germany, the United States, Sweden, The Netherlands, and the U.K. Information on temporal trends comes from the analysis of date able deposits (e.g., sediments), retrospective analysis of preserved or archived samples, and ongoing monitoring programs. The data on changes in air concentrations, deposition, sediments, soil, biota, food, and human tissues are reviewed. The evidence for natural input/formation of PCDD/Fs is also briefly reviewed and discussed. Human activity has dominated PCDD/F inputs to the environment this century. Conceptually, it is probably appropriate to consider a ‘pulse' of PCDD/Fs arising from human activities entering the environment in the 1930/1940s, peaking in the 1960/1970s, and continuing to a lesser degree today. A series of measures introduced in the past, recently, and anticipated for the future have reduced emissions to the atmosphere of industrialized countries and are projected to continue to reduce emissions over the coming decade. Comprehensive monitoring programs are required to confirm the interpretation of past changes and projected future declines presented here.