Counterfeiting is the largest criminal enterprise in the world. Counterfeit goods present two facets to forensic science: as evidence and as an insight into the classification of things as evidence. As evidence, counterfeits and their kin are the retail storefront of the criminal and terrorist worlds. It is estimated that the world trade in counterfeits is 3.3% of world trade (around USD 509 billion) and rising; nearly all of the proceeds fund organized crime or terrorism. Counterfeits are a danger not only to the economy and consumer goods but also to the end users of other types of goods; for example, the US Federal Aviation Commission estimates that over a half million airplane replacement parts are counterfeit. Counterfeits are a threat not only to the global economy but also to national security. There is no limit to what consumer goods can be counterfeited – from shoe polish to pharmaceuticals to construction materials. Given their prevalence, counterfeits are likely to be encountered as evidence in criminal or civil cases and forensic scientists need to be aware of them as a class of item. Implicit in the definition of a counterfeit is the definition of the legitimate version of said item. By studying counterfeits, forensic scientists can learn more about how authentic products are defined and produced, leading to higher quality analyses and interpretations.