A lay witness is a person who gives evidence in legal proceedings based on their own observations or experiences, rather than on specialist or expert knowledge. In UK and Irish practice, the term is descriptive rather than formally defined in a single statute, but is widely used in civil and criminal courts to distinguish such witnesses from expert witnesses.A lay witness may testify about facts they perceived (for example, what they saw, heard, or did), and may give limited opinion evidence where this is treated as a shorthand for factual observation (such as describing someone as “drunk” or “angry”). They must not provide technical or professional opinions that require expertise; that is the role of an expert witness.Across England and Wales, Scotland, Northern Ireland and Ireland, the core concept is consistent, although detailed evidential rules are found in different legislation and procedural codes (for example, the Criminal Justice Act 2003 in England and Wales, and equivalent rules of evidence elsewhere). The reliability, credibility and admissibility of lay witness evidence are central issues in trial preparation, examination-in-chief, cross-examination and fact-finding by the court.