Scientific evidence is information that has been developed through a process known as the "scientific method" -- meaning that the information is considered valid because it has been tested and shown to accurately describe what it purports to describe. Typically, scientific evidence has been published in journals, tested by other scientists and generally accepted as valid within the relevant scientific community. Common examples of scientific evidence include DNA analyses, hair and fiber comparisons, fingerprints and voice identification evidence. Because scientific evidence is by definition beyond the realm of judges' and jurors' everyday experiences, the prosecution and the defense use qualified expert witnesses to introduce scientific evidence into the courtroom.

There are, of course, rules about when and how scientific evidence may be used in court. If a scientific theory is well established, testimony from a qualified expert witness based on that theory is usually admissible at trial, without additional expert testimony regarding the reliability of the evidence. For example, an expert is seldom necessary to convince a judge of the validity of fingerprint analysis or radar speed testing devices. However, as novel scientific theories emerge, experts must convince judges that information based on these new ideas is reliable and therefore appropriate for consideration by the judge or jury.

To establish the reliability of scientific evidence, the party seeking to introduce the evidence ordinarily schedules a "mini-trial" in which an expert testifies and explains the scientific methodology involved. For example, to use DNA evidence for the purpose of identifying a suspect (by comparing samples taken from the suspect with samples found at the crime scene), some judges still require the prosecutor to establish the reliability of that evidence in a mini-trial. If the judge is then convinced of its reliability, DNA evidence can be used in that case. Most courts, however, now accept DNA evidence as reliable and do not require a foundation (the function of the mini-trial) for its use.