Governmental vehicle emission standards began in 1959 in California. The federal government became involved eight years later as Congress passed the Air Quality Act of 1967, which designated air quality regions throughout the country and gave states the responsibility for adopting and enforcing pollution control standards in those regions. In 1970, President Richard Nixon brought those responsibilities under one umbrella with the establishment of the Environmental Protection Agency. Since then, the EPA has regulated diesel fuel emissions on an almost annual basis.
Currently, engine manufacturers are operating under a 1998 consent decree that, among other things, mandated meeting the 2004 emission standards by October 2002. This "tier 1" regulation targeted the use of low sulphur diesel fuel.
Engine manufacturers began meeting stringent 2007 EPA diesel emission standards for diesel's most common pollutants -- Nitrogen Oxides (NOx), Particulate Matter (PM) and Hydrocarbons (HC) -- and reduced the levels to 0.20, 0.01 and .14 grams/brake-horsepower-hour, respectively, through the use of diesel oxidation catalysts and the introduction of ultra low sulphur diesel.
Even stricter engine "tier 3" standards went into place in 2010 that required diesel particular filters and Selective Catalyst Reduction.
See DieselNet and the EPA for more in-depth information regarding diesel emission standards. The U.S. Department of Energy's Alternative Fuels & Advanced Vehicles Data Center has a comprehensive list of federal and state laws.