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 Connecticut, evidence is regulated by both state statutes and rules of court, which align with the Federal Rules of Evidence to a significant extent. Evidence must be relevant to the issues in a case to be admissible in court, meaning it must have any tendency to make a fact more or less probable than it would be without the evidence and the fact must be of consequence in determining the action. Physical and digital evidence, such as written agreements and email communications, must be authenticated to be admitted. This involves demonstrating that the evidence is what the proponent claims it to be. Testimony from witnesses, including expert witnesses, is also subject to rules of admissibility. Expert witness testimony, in particular, must be based on sufficient facts or data and be the product of reliable principles and methods that the expert has applied reliably to the facts of the case. The judge acts as a gatekeeper in determining the reliability and admissibility of expert testimony. All evidence, to be considered by the judge or jury, must comply with these and other evidentiary rules, such as the prohibition against hearsay, unless an exception applies.