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 Illinois, evidence is crucial for resolving disputes in court and must be relevant to the issues at hand to be admissible. This includes both physical and digital evidence, such as contracts and emails, as well as witness testimony. The relevance of evidence is determined by whether it makes a fact more or less probable than it would be without the evidence and whether the fact is of consequence in determining the action. Expert witness testimony is subject to additional scrutiny; it must be provided by someone qualified by knowledge, skill, experience, training, or education, and the testimony must be based on sufficient facts or data and be the product of reliable principles and methods. The expert must also have applied the principles and methods reliably to the facts of the case. The judge in an Illinois court acts as the gatekeeper in determining the admissibility of evidence, including the reliability of expert testimony, in accordance with the Illinois Rules of Evidence and relevant case law.