International negotiations on transboundary air pollution present a long and rich set of case studies within the field of global environmental politics. Scholars have produced an extensive literature addressing the successes and failures of international attempts to address these problems. Transboundary flows of air pollution possess a number of characteristics that make these issues particularly interesting for scholars. For example while smog is clearly visible in urban areas, the most important transboundary air pollutants such as sulfur dioxide, nitrogen oxides, the most dangerous particulate matter, persistent organic pollutants (POPs), and volatile organic compounds (VOCs) are not clearly visible to the naked eye (see Chapter 32). The public is thus unlikely to independently identify these pollutants as problems. Instead, scientists must play a critical role in defining and framing the problems for political debate and policy response (see Chapter 17). The relationships among science, problem framing, and political responses have been a central focus of the international air pollution literature.