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 Missouri, evidence is governed by both state statutes and rules of court, which align with the principles of relevance and reliability. 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 that the fact is of consequence in determining the action. Missouri law also requires that evidence must not be overly prejudicial, confusing, or misleading. Expert witness testimony in Missouri is subject to the Daubert standard, following the adoption of amendments to the Missouri Rules of Civil Procedure, which requires that the testimony is based on sufficient facts or data, is the product of reliable principles and methods, and that the expert has reliably applied the principles and methods to the facts of the case. The judge acts as a gatekeeper in determining the admissibility of expert testimony. Digital evidence, such as emails, must be authenticated and shown to be reliable to be admissible. The Missouri Rules of Evidence provide the framework for the admissibility of all forms of evidence in court proceedings.