In almost every instance, scientific evidence tests the abilities of judges, lawyers, and jurors, all of whom may lack the scientific expertise to comprehend the evidence and evaluate it in an informed manner. Forensic science experts and evidence are routinely used in the service of the criminal justice system. DNA testing may be used to determine whether sperm found on a rape victim came from an accused party; drug analysis may be used to determine whether pills found in a person’s possession were illicit; a latent fingerprint found on a gun may be used to determine whether a defendant handled the weapon; and an autopsy may be used to determine the cause of death of a murder victim. In order for qualified forensic science experts to testify competently about forensic evidence, they must first find the evidence in a usable state and properly preserve it. A latent fingerprint that is badly soiled when found cannot be usefully saved, analyzed, or explained. An inadequate drug sample may be insufficient to allow for proper analysis. And, DNA tests performed on a contaminated or otherwise compromised sample cannot reliably identify or eliminate an individual as the perpetrator of a crime. These are important matters associated with the proper “processing” of forensic evidence. The goal of law enforcement actions is to identify those who have committed crimes and to prevent the criminal justice system from erroneously convicting the innocent. So it matters a great deal whether an expert is qualified to testify about forensic evidence and whether the evidence is sufficiently reliable to merit a fact finder’s reliance on the truth that it purports to support.