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 Ohio, evidence is regulated by both state statutes and rules of court, particularly the Ohio Rules of Evidence. These rules dictate what is considered admissible evidence in court. Evidence must be relevant, 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 admissible. Testimony from witnesses is also subject to rules of evidence, including the requirement that witnesses have personal knowledge of the facts to which they testify. Expert witness testimony is subject to additional scrutiny; it must be provided by a qualified expert, and the testimony 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 in the case has the role of gatekeeper in determining the admissibility and reliability of evidence presented in court.