Loading
State by State Expert Witness Rules
8 min read
Published Date:
Last Modified Date:
New York expert witness disclosure requirements vary between general state-court practice under CPLR § 3101(d)(1)(i) and the more structured report and scheduling requirements of Commercial Division Rule 13(c). This article explains disclosure content, timing, preclusion risks, good-cause considerations, and the differing Frye and Daubert standards that can affect expert testimony in New York litigation.
Expert disclosure in New York is easier to manage when you treat it like case management, not a last-minute exchange. The main reason is simple: New York litigators operate under two different expert disclosure frameworks. They can feel similar at first glance, but they impose different requirements and timelines depending on the forum.
One framework is the general state-court rule in CPLR § 3101(d)(1)(i). The other applies in the Commercial Division and comes from Commercial Division Rule 13(c) under 22 NYCRR 202.70.
Part 202 contains the Uniform Civil Rules, including the Commercial Division rules in § 202.70, which is where Rule 13(c) sits.
This guide focuses on what the rules require and, as a practical matter, where timing and disclosure quality can create motion risk.
In most New York state-court matters outside the Commercial Division, CPLR § 3101(d)(1)(i) governs expert disclosure. Upon request, a party must identify each expert it expects to call at trial and provide information in reasonable detail, including:
Limited malpractice exception: In medical, dental, and podiatric malpractice cases, a party responding to a request may omit the expert’s name while still disclosing the other required information. Any party may also make a written offer, served on all parties and filed with the court, to disclose the expert’s name and produce the expert for a deposition. The offer must be accepted by all parties, and a failure to respond within 20 days is treated as a rejection.
The key point is what CPLR 3101(d)(1)(i) actually says. It requires disclosure in reasonable detail, but it does not impose a federal-style written report requirement as the default rule. Courts can still order additional disclosure in appropriate circumstances, but the CPLR baseline starts from the “reasonable detail” notice framework.
In the Commercial Division of the New York Supreme Court, Rule 13(c) (22 NYCRR 202.70.13[c]) sets a structured approach to expert disclosure. It requires the parties to confer on a schedule for expert disclosure, complete expert disclosure within a defined window tied to the close of fact discovery, and exchange reports and take depositions of testifying experts on that schedule.
A simple way to think about the difference is this: CPLR 3101(d) tells you what must be disclosed in reasonable detail upon request, while Rule 13(c) adds a schedule and a report-based structure for the Commercial Division.
Expert disclosure in New York is easier to manage when you treat it like case management, not a last-minute exchange. The main reason is simple: New York litigators operate under two different expert disclosure frameworks. They can feel similar at first glance, but they impose different requirements and timelines depending on the forum.
One framework is the general state-court rule in CPLR § 3101(d)(1)(i). The other applies in the Commercial Division and comes from Commercial Division Rule 13(c) under 22 NYCRR 202.70.
Part 202 contains the Uniform Civil Rules, including the Commercial Division rules in § 202.70, which is where Rule 13(c) sits.
This guide focuses on what the rules require and, as a practical matter, where timing and disclosure quality can create motion risk.
In most New York state-court matters outside the Commercial Division, CPLR § 3101(d)(1)(i) governs expert disclosure. Upon request, a party must identify each expert it expects to call at trial and provide information in reasonable detail, including:
Limited malpractice exception: In medical, dental, and podiatric malpractice cases, a party responding to a request may omit the expert’s name while still disclosing the other required information. Any party may also make a written offer, served on all parties and filed with the court, to disclose the expert’s name and produce the expert for a deposition. The offer must be accepted by all parties, and a failure to respond within 20 days is treated as a rejection.
The key point is what CPLR 3101(d)(1)(i) actually says. It requires disclosure in reasonable detail, but it does not impose a federal-style written report requirement as the default rule. Courts can still order additional disclosure in appropriate circumstances, but the CPLR baseline starts from the “reasonable detail” notice framework.
In the Commercial Division of the New York Supreme Court, Rule 13(c) (22 NYCRR 202.70.13[c]) sets a structured approach to expert disclosure. It requires the parties to confer on a schedule for expert disclosure, complete expert disclosure within a defined window tied to the close of fact discovery, and exchange reports and take depositions of testifying experts on that schedule.
A simple way to think about the difference is this: CPLR 3101(d) tells you what must be disclosed in reasonable detail upon request, while Rule 13(c) adds a schedule and a report-based structure for the Commercial Division.
CPLR § 3101(d)(1)(i) sets what must be disclosed, but it does not specify a single statewide date for expert disclosure. In many cases, the schedule is set through the court’s case management and scheduling orders.
Practical takeaway: If you are building your expert plan, anchor it to the dates the court set for the case. That reduces the risk of disputes about late disclosure or inadequate preparation time.
Late expert disclosure is not automatically fatal. Martin illustrates that courts may look closely at whether a late CPLR 3101(d)(1)(i) disclosure was willful or prejudicial when deciding whether preclusion is appropriate.
In plain terms, “willful” usually means the delay looks avoidable rather than unavoidable. “Prejudicial” generally means the timing meaningfully limits the other side’s ability to respond in a fair way.
Martin also shows one way a court may address claimed prejudice, including allowing voir dire of the expert to reduce surprise, especially where no adjournment is requested.
CPLR 3101(d)(1)(i) addresses the situation where an expert is retained too close to trial to give earlier notice. If a party shows good cause for retaining an expert too close to trial to give appropriate notice, the party is not precluded solely for that timing noncompliance, and the court may make whatever order is just.
What makes a “good cause” showing persuasive is usually specificity. Even when the reason is legitimate, the record should be clear about what changed, why earlier retention was not feasible, and what steps can reduce any resulting unfairness.
In Commercial Division cases, Rule 13(c) generally requires a written, signed expert report for retained or specially employed experts, and for certain employee experts whose duties regularly involve giving expert testimony, unless the parties stipulate or the court orders otherwise.
A compliant report is expected to disclose at least the following:
Rule 13(c) is built around completing expert disclosure on a defined schedule. In general terms, it requires:
If a party objects to the procedure or timetable, the rule directs the parties to request a conference with the court.
Rule 13(c) also includes a direct consequence: expert disclosure provided after the rule’s deadlines, without good cause shown, will be precluded from use at trial.
The rule text does not use labels like “method drift” or “fit analysis,” but the practical risk is straightforward. When the report does not clearly lay out opinions, bases, and what was considered, an opponent can more easily argue that the disclosure is incomplete or that the timing creates unfair surprise. As a practical matter, that can strengthen motions to limit opinions or seek preclusion.
Supplementation is safest when it completes or clarifies what was already disclosed, not when it introduces a new theory after the other side has built its discovery plan around the original report.
In New York state court, Frye is the current standard for evaluating certain scientific expert evidence, with the focus on general acceptance of the methodology in the relevant scientific community.
Frye is not only about credentials. When Frye is in play, the key question is whether the method itself has achieved general acceptance in the relevant field.
Parker illustrates that Frye issues can overlap with more basic foundation questions, including whether accepted methods were applied in a way that supports the opinion.
Even where the dispute is not framed as Frye, courts still examine whether an expert opinion is grounded in the record and supported by a reliable reasoning chain. That reliability-and-fit concept is explicit in federal practice under Rule 702. As a practical matter, it is also a useful framework for evaluating expert work product in New York cases.
Federal courts apply Daubert’s Rule 702 framework, which directs the judge to assess expert testimony for reliability and relevance. Rule 702 places the admissibility showing on the proponent and requires the court to assess reliability and application.
Because forum can shape how reliability challenges are argued, many litigators treat the expert’s reasoning chain, data inputs, and assumptions as something that should be defensible from day one, not something to reconstruct after a motion is filed.
Daubert also emphasizes that the admissibility inquiry is flexible and should focus on principles and methodology, not the conclusion the expert wants to reach.
These are practice guidelines, not legal requirements.
Best practice in CPLR cases:
Best practice in Commercial Division cases:
Do New York state courts require full written expert reports?
Usually no outside the Commercial Division. CPLR § 3101(d)(1)(i) generally requires disclosure in reasonable detail upon request, rather than a full expert report.
In New York malpractice cases, can you withhold the expert’s name?
In medical, dental, and podiatric malpractice matters, CPLR § 3101(d)(1)(i) allows the name to be omitted in the initial response. The statute also provides an offer-and-response procedure that can lead to disclosure of the name and a deposition if all parties accept the offer.
When must expert disclosure be completed in the Commercial Division?
Rule 13(c) requires the parties to confer on an expert schedule no later than 30 days before fact discovery is completed, and it requires expert disclosure to be completed no later than four months after fact discovery is completed unless the court orders otherwise. The note of issue and certificate of readiness may not be filed until expert disclosure is complete.
Can late expert disclosure lead to preclusion in New York?
Yes. In CPLR cases, courts may focus on willfulness and prejudice when deciding whether preclusion is appropriate, as illustrated in Martin. In the Commercial Division, Rule 13(c) states that expert disclosure provided after the rule’s deadlines, without good cause shown, will be precluded.
Does New York use Frye or Daubert?
New York state courts apply Frye for certain scientific testimony involving novel methods. Federal courts apply Daubert through Federal Rule of Evidence 702 for federal trials.
CPLR § 3101(d)(1)(i) sets what must be disclosed, but it does not specify a single statewide date for expert disclosure. In many cases, the schedule is set through the court’s case management and scheduling orders.
Practical takeaway: If you are building your expert plan, anchor it to the dates the court set for the case. That reduces the risk of disputes about late disclosure or inadequate preparation time.
Late expert disclosure is not automatically fatal. Martin illustrates that courts may look closely at whether a late CPLR 3101(d)(1)(i) disclosure was willful or prejudicial when deciding whether preclusion is appropriate.
In plain terms, “willful” usually means the delay looks avoidable rather than unavoidable. “Prejudicial” generally means the timing meaningfully limits the other side’s ability to respond in a fair way.
Martin also shows one way a court may address claimed prejudice, including allowing voir dire of the expert to reduce surprise, especially where no adjournment is requested.
CPLR 3101(d)(1)(i) addresses the situation where an expert is retained too close to trial to give earlier notice. If a party shows good cause for retaining an expert too close to trial to give appropriate notice, the party is not precluded solely for that timing noncompliance, and the court may make whatever order is just.
What makes a “good cause” showing persuasive is usually specificity. Even when the reason is legitimate, the record should be clear about what changed, why earlier retention was not feasible, and what steps can reduce any resulting unfairness.
In Commercial Division cases, Rule 13(c) generally requires a written, signed expert report for retained or specially employed experts, and for certain employee experts whose duties regularly involve giving expert testimony, unless the parties stipulate or the court orders otherwise.
A compliant report is expected to disclose at least the following:
Rule 13(c) is built around completing expert disclosure on a defined schedule. In general terms, it requires:
If a party objects to the procedure or timetable, the rule directs the parties to request a conference with the court.
Rule 13(c) also includes a direct consequence: expert disclosure provided after the rule’s deadlines, without good cause shown, will be precluded from use at trial.
The rule text does not use labels like “method drift” or “fit analysis,” but the practical risk is straightforward. When the report does not clearly lay out opinions, bases, and what was considered, an opponent can more easily argue that the disclosure is incomplete or that the timing creates unfair surprise. As a practical matter, that can strengthen motions to limit opinions or seek preclusion.
Supplementation is safest when it completes or clarifies what was already disclosed, not when it introduces a new theory after the other side has built its discovery plan around the original report.
In New York state court, Frye is the current standard for evaluating certain scientific expert evidence, with the focus on general acceptance of the methodology in the relevant scientific community.
Frye is not only about credentials. When Frye is in play, the key question is whether the method itself has achieved general acceptance in the relevant field.
Parker illustrates that Frye issues can overlap with more basic foundation questions, including whether accepted methods were applied in a way that supports the opinion.
Even where the dispute is not framed as Frye, courts still examine whether an expert opinion is grounded in the record and supported by a reliable reasoning chain. That reliability-and-fit concept is explicit in federal practice under Rule 702. As a practical matter, it is also a useful framework for evaluating expert work product in New York cases.
Federal courts apply Daubert’s Rule 702 framework, which directs the judge to assess expert testimony for reliability and relevance. Rule 702 places the admissibility showing on the proponent and requires the court to assess reliability and application.
Because forum can shape how reliability challenges are argued, many litigators treat the expert’s reasoning chain, data inputs, and assumptions as something that should be defensible from day one, not something to reconstruct after a motion is filed.
Daubert also emphasizes that the admissibility inquiry is flexible and should focus on principles and methodology, not the conclusion the expert wants to reach.
These are practice guidelines, not legal requirements.
Best practice in CPLR cases:
Best practice in Commercial Division cases:
Do New York state courts require full written expert reports?
Usually no outside the Commercial Division. CPLR § 3101(d)(1)(i) generally requires disclosure in reasonable detail upon request, rather than a full expert report.
In New York malpractice cases, can you withhold the expert’s name?
In medical, dental, and podiatric malpractice matters, CPLR § 3101(d)(1)(i) allows the name to be omitted in the initial response. The statute also provides an offer-and-response procedure that can lead to disclosure of the name and a deposition if all parties accept the offer.
When must expert disclosure be completed in the Commercial Division?
Rule 13(c) requires the parties to confer on an expert schedule no later than 30 days before fact discovery is completed, and it requires expert disclosure to be completed no later than four months after fact discovery is completed unless the court orders otherwise. The note of issue and certificate of readiness may not be filed until expert disclosure is complete.
Can late expert disclosure lead to preclusion in New York?
Yes. In CPLR cases, courts may focus on willfulness and prejudice when deciding whether preclusion is appropriate, as illustrated in Martin. In the Commercial Division, Rule 13(c) states that expert disclosure provided after the rule’s deadlines, without good cause shown, will be precluded.
Does New York use Frye or Daubert?
New York state courts apply Frye for certain scientific testimony involving novel methods. Federal courts apply Daubert through Federal Rule of Evidence 702 for federal trials.
State by State Expert Witness Rules
State by State Expert Witness Rules