What Is an Expert Report? Contents, Deadlines, and Who Must File

An expert report is a written document, prepared and signed by a retained expert witness, that sets out every opinion the expert will offer at trial along with the facts, data, and methods behind those opinions. In federal civil cases, Federal Rule of Civil Procedure 26(a)(2)(B) requires one from any witness who is retained to give expert testimony or whose job regularly involves it, and the report must also include the expert’s qualifications, recent publications, recent testimony, and compensation.¹ The purpose is simple: put the other side on notice of what the expert will say, in enough detail that they can prepare cross-examination, retain their own expert, or challenge the testimony before trial.

What Has to Be in the Report

Rule 26(a)(2)(B) requires the expert, personally, to sign a written report containing each of the following:¹

A complete statement of every opinion the expert will offer at trial, together with the reasons for each opinion and the facts or data the expert considered in forming them.
Any exhibits the expert will use to summarize or support those opinions, including charts, graphs, and models.
The expert’s qualifications, along with a list of all publications authored in the previous ten years.
A list of every case in which the expert has testified at trial or by deposition during the previous four years.
A statement of the compensation the expert is being paid for the analysis and the testimony.

Two features of this list often surprise people the first time they prepare a report. The report has to be the expert’s own work, not something ghostwritten by counsel and signed off on, and the opinions in it have to be complete. If the expert later offers an opinion at trial that was not disclosed in the report, opposing counsel can object and the court may strike it. The compensation and prior-testimony disclosures exist because both bear on credibility, and the opposing side is entitled to use them.

When the Report Is Due

Under Rule 26(a)(2)(D), expert disclosures must be made at least 90 days before the trial date or the date the case must be ready for trial, unless the court or the parties set a different schedule. A rebuttal expert, offered solely to contradict or rebut the other side’s expert, must be disclosed within 30 days after that other disclosure.²

Those are the defaults. In most federal cases, the court issues a scheduling order early on that sets its own expert deadlines, and those court-ordered dates control. Treat them as hard. Once the report is served, opposing counsel has the right to depose the expert, but the deposition cannot happen until the report is in hand.²

Who Has to Write One

The written-report requirement applies to experts who are retained or specially employed to provide expert testimony, and to employees whose regular duties involve giving expert testimony. If a party has designated someone to testify as an expert and that person falls into either category, the report is mandatory.¹

Consulting experts are different. Attorneys often hire specialists purely to help them understand a technical area, size up the case, or prepare questions for the other side’s expert. A pure consulting expert does not testify, does not write a Rule 26 report, and their work is generally shielded from discovery. That protection largely disappears if the consultant is later designated as a testifying expert, at which point the report requirement attaches.²

Qualification to serve as an expert at all is governed by Federal Rule of Evidence 702, which looks at knowledge, skill, experience, training, or education. No specific degree or license is required; a mechanic with decades of hands-on experience can qualify as readily as a credentialed engineer, provided the expertise helps the court understand the evidence or resolve a factual dispute.³

How Courts Judge the Opinions Inside the Report

The report is also the document a court reads when deciding whether the expert’s testimony is admissible at all. Rule 702 requires that the testimony rest on sufficient facts or data, use reliable principles and methods, and apply those methods reliably to the facts of the case. A 2023 amendment added that the court must find it “more likely than not” that each requirement is met, confirming that reliability is a threshold the judge enforces rather than an issue for the jury.³

On top of Rule 702 sits the Supreme Court’s 1993 decision in Daubert v. Merrell Dow Pharmaceuticals, which casts the trial judge as a gatekeeper for scientific and technical testimony. Courts assessing a methodology look at whether it has been tested, whether it has been subjected to peer review and publication, its known or potential error rate and the standards controlling its operation, and whether it is generally accepted in the relevant community. These are guidelines rather than a checklist, and the focus is on the soundness of the methods rather than on the conclusions they produce.⁴

This is why the written report matters so much. A report that clearly identifies the data relied on, explains the method, and shows the analytical steps gives the judge a basis to admit the testimony. A report that jumps from data to conclusion without showing the work is exactly what a Daubert motion is built to attack. Either side can move to exclude the opposing expert, and the report is the primary document the court will read in deciding.

What Happens If the Report Is Late or Incomplete

Federal Rule of Civil Procedure 37(c)(1) sets a severe default sanction. If a party fails to provide information or identify a witness as required by Rule 26(a), the party is not allowed to use that information or witness to supply evidence on a motion, at a hearing, or at trial. The only way out is to show that the failure was substantially justified or harmless, and courts read that exception narrowly.⁵

The court can also, or instead, order the non-disclosing party to pay the other side’s reasonable expenses and attorney’s fees caused by the failure, inform the jury of the failure to disclose, and impose any other appropriate sanction, including the sanctions available for disobeying a discovery order, which can reach as far as dismissing claims or entering a default judgment.⁵

Exclusion is the sanction that matters most in practice. In many cases, a party cannot prove causation or damages without expert testimony. If that testimony is excluded because the report was late, unsigned, or missing a required piece, the underlying claim often collapses with it.

State Court Cases Follow Different Rules

Everything above is federal. State courts have their own procedures for expert disclosures and their own standards for admissibility. Some states follow Daubert; others use the older Frye standard, which asks only whether the methodology is generally accepted in the relevant scientific community; a few use hybrid approaches. The required contents and timing of expert reports also vary, with some states demanding less than the federal rules and others adding obligations like certifications or supplemental disclosures. If the case is in state court, that jurisdiction’s rules control, and the federal framework described here is a reference point rather than binding authority.

1
Cornell Law School Legal Information Institute. Federal Rules of Civil Procedure Rule 26 – Duty to Disclose; General Provisions Governing Discovery – Section: Disclosure of Expert Testimony
2
Cornell Law School Legal Information Institute. Federal Rules of Civil Procedure Rule 26 – Duty to Disclose; General Provisions Governing Discovery
3
Legal Information Institute. Federal Rules of Evidence Rule 702 – Testimony by Expert Witnesses
4
Justia Law. Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993)
5
Legal Information Institute. Federal Rules of Civil Procedure Rule 37 – Failure to Make Disclosures or to Cooperate in Discovery