Evidence is information that exists in physical or digital form—such as written agreements and e-mail communications—as well as testimony from witnesses who have personal knowledge of facts relevant to a dispute. Evidence generally must be relevant to be admitted in court and considered by the factfinder (judge or jury) in resolving a lawsuit. And some forms of evidence, such as expert witness testimony, must be shown to be sufficiently reliable (as determined by the judge) to be admitted in evidence.

In Massachusetts, evidence is regulated by both state statutes and the Massachusetts Guide to Evidence. Evidence must be relevant to the issues in a legal dispute to be admissible in court. Relevance is determined by whether the evidence makes a fact more or less probable than it would be without the evidence, and if that fact is of consequence in determining the action. Physical and digital evidence, such as written agreements and email communications, must be authenticated to be admitted. Testimony from witnesses with personal knowledge of relevant facts can also be presented as evidence. Expert witness testimony is subject to a reliability standard, which is assessed by the judge. The judge determines whether the expert's methodology is sufficiently reliable and relevant to the case at hand before allowing the testimony to be considered by the factfinder, which can be either a judge or a jury. The rules governing evidence in Massachusetts are designed to ensure that the factfinder receives information that is both pertinent and trustworthy to make a fair and informed decision.