Patent Opinion Letters: Non-Infringement, Invalidity, and What Makes Them Hold Up
A patent opinion letter is a written analysis, prepared by a patent attorney, of whether a specific product or process infringes a specific patent, whether that patent is valid, or both. Companies usually ask for one after a demand letter arrives, before a product launch, or before an acquisition or investment that depends on a technology being clear to use.
This article explains what opinions are for, the main types, where the deciding facts usually sit, and what separates an opinion that holds up from one that does not.
Why opinions still matter
Since the America Invents Act, federal law has provided that failing to obtain advice of counsel, or failing to present it in court, cannot be used to prove willful infringement or intent to induce infringement (35 U.S.C. § 298). An opinion is not mandatory.
It is still valuable, for three reasons:
Enhanced damages. Under Halo Electronics v. Pulse Electronics (2016), courts may award up to three times actual damages for egregious, willful infringement, and the question turns on what the accused infringer knew and believed at the time. A timely, well-reasoned opinion is strong evidence of a good-faith belief.
Induced infringement. A good-faith belief that a product does not infringe can negate the intent required for inducement. A belief that the patent is invalid cannot (Commil USA v. Cisco Systems, 2015). That distinction often determines which kind of opinion a company needs.
Better decisions. Most importantly, an opinion tells you what to do next: continue, redesign, negotiate a license, or challenge the patent, and how much risk each path carries.
The main types of opinion
Non-infringement
A non-infringement opinion construes the patent's claims and compares every element of each relevant claim to the product as it is actually built. A product infringes only if it meets every element of a claim, either literally or under the doctrine of equivalents. The opinion addresses both.
The doctrine of equivalents is often where the analysis is decided. Narrowing amendments and arguments made during prosecution can bar a patent owner from recapturing, through equivalents, what was given up to get the patent allowed (Festo v. Shoketsu, 2002).
Invalidity
An invalidity opinion evaluates whether the claims should have issued at all. The usual grounds are anticipation and obviousness over prior art (35 U.S.C. §§ 102 and 103, and KSR v. Teleflex, 2007), and failures of written description, enablement, or definiteness under § 112 (Nautilus v. Biosig, 2014). A good invalidity opinion identifies specific references, maps them to each claim element, and explains why a skilled person would have combined them.
In court, a patent is presumed valid and invalidity must be proven by clear and convincing evidence (Microsoft v. i4i, 2011). An invalidity opinion should be written with that standard in mind.
Freedom-to-operate and clearance
A freedom-to-operate opinion looks forward. Instead of analyzing one patent someone has asserted, it searches for in-force patents that a planned product might infringe, then analyzes the ones that matter. It is most useful before launch, when design changes are still cheap.
Design-around and unenforceability
Some opinions evaluate a proposed redesign, confirming that a modified product avoids the claims. Less often, an opinion addresses unenforceability, such as inequitable conduct before the Patent Office, which requires proof of both but-for materiality and specific intent to deceive (Therasense v. Becton Dickinson, 2011).
In practice, many opinions combine non-infringement and invalidity analysis, so the client has more than one basis for its position.
The answer is usually in the file history
The claims of a patent mean what the patent, its claims, and its prosecution history say they mean (Phillips v. AWH, 2005). That makes the file history, the full record of exchanges between the applicant and the Patent Office, central to almost every opinion.
The file history shows which claim terms the applicant narrowed to get around prior art, which arguments it made to distinguish references, and what it disclaimed along the way. A clear disclaimer during prosecution can limit claim scope even if the claim language looks broad (Omega Engineering v. Raytek, 2003).
This gets harder with complex families: continuations, continuations-in-part, reissues, and patents that have been through post-grant proceedings. Statements made in a parent application can carry into its children, and statements made in later proceedings before the Patent Office can narrow the claims too (Aylus Networks v. Apple, 2017). Reading these records correctly means reviewing every related file, not just the one patent in the demand letter. Hands-on experience in them is uncommon, even among patent attorneys.
An opinion is only as strong as its reading of the file history.
What a competent opinion looks like
An opinion is only as useful as its reasoning. Courts and juries give little weight to conclusory opinions. A strong one:
Comes from a registered patent attorney, as an opinion of counsel.
Reviews the complete file history, including related applications and any reexamination or post-grant record.
Construes the disputed claim terms, starting from the patent itself and its prosecution history.
Analyzes each relevant claim element by element, including dependent claims.
Is based on the actual product (engineering documents, samples, or source code where relevant), not marketing descriptions.
Addresses the doctrine of equivalents, not only literal infringement.
For invalidity, maps specific prior art to each element and explains any combination.
Is candid about weaknesses, so the client can weigh real risk.
Is obtained promptly after notice of the patent, while it can still guide decisions.
Privilege: decide later, but plan now
An opinion is generally privileged. If a company later relies on it to defend against willfulness, it waives privilege over the subject matter of the opinion. Courts have generally declined to extend that waiver to separate trial counsel (In re Seagate, 2007), which is one reason many companies keep opinion counsel separate from litigation counsel. The decision to rely on an opinion can be made later; what matters early is that the opinion be written so that it could be relied on.
When to get one
After a demand letter or cease-and-desist arrives, before responding on the merits. See our guide to the first 72 hours after a demand letter.
Before launching a product into a space where competitors hold patents.
Before acquiring a company or investing in a technology whose value depends on freedom to operate.
When a competitor's patent issues or a continuation is allowed that may reach your product.
When you are considering a redesign and need to know whether it actually avoids the claims.
How we approach opinions
Patent Intelligence Group prepares non-infringement, invalidity, and freedom-to-operate opinions led by Jennifer Meredith, a registered patent attorney with 25 years of experience. Every opinion starts with the complete file history and the actual product, and ends with a clear, reasoned conclusion you can act on.
If you have received a demand letter, start with our patent demand letter page. For opinions generally, see Patent Opinions, or request a confidential consultation.
This article is general information, not legal advice, and does not create an attorney-client relationship. Every opinion depends on the specific patent, product, and record.
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