Black carbon (or soot) is a potent climate forcer that may rank second only to CO2 in warming the planet. Reducing black carbon emissions could avoid as much as .5°C (or nearly 1° Fahrenheit) of warming. In the United States, the major sources of black carbon emissions are diesel combustion from the transportation sector, diesel combustion from stationary sources, coal combustion, and biomass combustion. The chapter discusses key strategies and obstacles to reducing black carbon emissions, with a particular focus on reducing diesel fuel use. The chapter explains how several CAA programs that regulate emissions of fine particulate matter (PM), including the mobile source standards program, several stationary source programs, national ambient air quality standards, and SIPS, could help reduce or eliminate black carbon emissions. The chapter also examines programs that state and local governments could use independent of the CAA to reduce black carbon emissions. Finally, the chapter identifies obstacles to successful regulation, proposes strategies to overcome obstacles, identifies areas of uncertainty, and briefly identifies measures discussed in other chapters in the book that would complement or accelerate the black carbon reduction strategies identified here.