A witness is someone who has firsthand knowledge about a crime or dramatic event through their senses (e.g. seeing, hearing, smelling, touching), and can help certify important considerations to the crime or event. A witness who has seen the event firsthand is known as an "eye-witness". Witnesses are often called before a court of law to testify in trials.

A subpoena commands a person to appear. In many jurisdictions it is compulsory to comply, to take an oath, and tell the truth, under penalty of perjury. It is used to compel the testimony of witnesses in a trial. Usually it can be issued by a judge or by the lawyer representing the plaintiff or the defendant in a civil trial or by the prosecutor or the defense attorney in a criminal proceeding.

A witness who specializes in an area of study relevant to the crime is called an expert witness. Scientists and doctors are often called to give expert witness testimony.

Eyewitness testimony

main: Eyewitness testimony

Eyewitness testimony is generally presumed to be better than circumstantial evidence. Studies have shown, however, that individual, separate witness testimony is often flawed, and parts of it can be meaningless. This can occur because of a person's faulty observation and recollection, because of a person's bias, or because the witness is lying. If several people witness a crime it is probative to look for similarities in their collective descriptions to substantiate the facts of an event, keeping in mind the contrasts of individual descriptions.

One study involved an experiment in which subjects acted as jurors in a criminal case. Jurors heard a description of a robbery-murder, then a prosecution argument, then an argument for the defense. Some jurors heard only circumstantial evidence, others heard from a clerk who claimed to identify the defendant. In the first case, 18% percent found the defendant guilty, but in the second, 72% found the defendant guilty (Loftus 1988). Lineups, where the eyewitness picks out a suspect from a group of people in the police station, are often grossly suggestive, and give the false impression that the witness remembered the suspect. In another study, students watched a staged crime. An hour later they looked through photos. A week later they were asked to pick the suspect out of lineups. 8% of the people in the lineups were mistakenly identified as criminals. 20% of the innocent people whose photographs were included were mistakenly identified (University of Nebraska 1977). Weapon focus effects in which the presence of a weapon impairs memory for surrounding details is also an issue.

Another study looked at sixty-five cases of "erroneous criminal convictions of innocent people." In 45% of the cases, eyewitness mistakes were responsible (Borchard p. 367).

The formal study of eyewitness memory is usually undertaken within the broader category of cognitive processes — the different ways in which we make sense of the world around us. We do this by employing the mental skills at our disposal such as thinking, perception, memory, awareness, reasoning and judgment. Although cognitive processes can only be inferred and cannot be seen directly, they all have very important practical implications within a legal context.