Establishing chain of custody is made of both a chronological and logical procedure, especially important when the evidence consists of fungible goods. In practice, this most often applies to illegal drugs which have been seized by law enforcement personnel. In such cases, the defendant at times disclaims any knowledge of possession of the controlled substance in question. Accordingly, the "chain of custody" documentation and testimony is presented by the prosecution to establish that the substance in evidence was in fact in the possession of the defendant.