In September 2015, the US Environmental Protection Agency (US EPA) issued a notice of violation of the Clean Air Act to the automobile manufacturer Volkswagen.1 It alleges that four-cylinder Volkswagen and Audi diesel vehicles from the years 2009–2015 included software that circumvented accurate emissions testing for certain air pollutants—in particular, nitrogen oxides (NOx). The US EPA and the California Air Resources Board determined that such vehicles emitted up to 40 times more NOx than current emission standards allow. It is estimated that there are a total of 11 million affected vehicles worldwide.