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 Wisconsin, as in other jurisdictions, evidence must be relevant 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 whether that fact is of consequence in determining the action. Both physical and digital forms of evidence, such as written agreements and email communications, can be admitted as long as they meet the relevance criteria. Testimony from witnesses with personal knowledge of relevant facts can also be considered. Expert witness testimony is subject to additional scrutiny; it must be shown to be reliable based on a standard set by the judge, which often involves the application of Daubert standards in federal court and corresponding state standards that assess the methodology and principles underlying the expert's opinion. Wisconsin courts may also refer to the Wisconsin Rules of Evidence, which are similar to the Federal Rules of Evidence, for guidance on the admissibility of evidence.