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 New York, evidence must be relevant and material to the issues being decided in a case to be admissible in court. Relevance means that the evidence must have any tendency to make a fact more or less probable than it would be without the evidence, and that the fact is one of consequence in determining the action. Materiality refers to the importance of the evidence in proving the point at issue. Physical evidence, such as written agreements, and digital evidence, like email communications, are subject to the same standards of admissibility as other types of evidence. Witness testimony, including that from expert witnesses, must be based on personal knowledge and is subject to reliability standards. Expert witness testimony, in particular, must be based on a reliable foundation and the expert must be qualified by knowledge, skill, experience, training, or education to testify on the subject. The judge acts as the gatekeeper in determining the admissibility of evidence, including the reliability of expert testimony, which is assessed under the Daubert standard or the Frye standard, depending on the type of case and the specific court's precedents.