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 around $250 billion or more and 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 type 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 the 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.