What Is Prior Art and How Do You Search It?

Prior art is any evidence that your invention — or something substantially similar — was already publicly known before the date you filed your patent application. If prior art exists for your invention, it cannot be patented. Understanding what counts as prior art, where to find it, and how to search it systematically is one of the most critical skills any inventor can develop.

Why Prior Art Matters

The patent system grants exclusive rights only to genuinely new inventions. If an idea was already public — in any form, anywhere in the world — it belongs to the public domain. No one person can monopolize what is already known.

Prior art serves two essential functions in this system:

Before filing: A thorough prior art search tells you whether your invention is patentable, helps you understand where to draw your claims, and prevents you from investing thousands of dollars in an application that will be rejected.

After filing: Patent examiners conduct their own searches. If they find prior art that anticipates your claims, they will reject your application. Courts can also use prior art to invalidate a granted patent if infringement is later disputed.

Many inventors skip the prior art search to save time or money. This is almost always a costly mistake.

What Counts as Prior Art

Prior art is broader than most inventors expect. It is not limited to other patents. Anything that makes your invention public before your filing date can qualify.

Granted patents and published patent applications — the most obvious form. Patents from any country, in any language, can be prior art.

Academic and scientific literature — journal articles, conference papers, PhD theses, and research reports. A publication from a university laboratory in South Korea that describes the same principle as your invention counts.

Products and commercial offerings — if a product incorporating your invention was sold anywhere in the world, it is prior art. Catalogs, product manuals, and advertisements count.

Trade shows and public demonstrations — if your invention (or a similar one) was demonstrated publicly, it is prior art.

Online content — YouTube videos, blog posts, forum discussions, and social media posts can all qualify, provided they are publicly accessible and dated.

Your own prior disclosures — if you publicly described your invention before filing, you may have created prior art against yourself. Most countries offer no grace period for this; the US offers a 12-month grace period for the inventor's own disclosures. Do not rely on grace periods — file first, then disclose.

Oral disclosures — in some jurisdictions, a public presentation or lecture can constitute prior art, though this is harder to prove without documentation.

The Two Types of Prior Art Rejection

When a patent examiner cites prior art against your application, they typically do so in one of two ways:

Anticipation (§102 in the US) — a single prior art document discloses every element of your claim. Your invention is exactly what was already described somewhere. The rejection is strong and typically requires amending claims rather than just arguing.

Obviousness (§103 in the US) — no single document describes your invention, but a combination of prior art references makes your invention obvious to someone skilled in the field. This is the more subjective — and more commonly disputed — of the two. A skilled attorney can often overcome obviousness rejections through argument.

Understanding which type you face determines your response strategy.

How to Conduct a Prior Art Search

A thorough prior art search is not a single Google search. It is a structured process across multiple databases, using multiple search strategies.

Step 1: Define Your Invention Precisely

Before searching, write a clear statement of what is genuinely new about your invention. Break it down into:

• The core function (what does it do?)
• The key structural features (how does it do it?)
• The problem it solves
• The field it belongs to (what industry or technology area?)

This clarity prevents you from searching too broadly (wasting time) or too narrowly (missing relevant prior art).

Step 2: Identify Keywords and Classifications

Patents are organized by classification systems that describe what an invention does, not just what it is called. The two main systems are:

CPC (Cooperative Patent Classification) — used by the USPTO, EPO, and most major offices. Hierarchical codes that categorize inventions by technology field.

IPC (International Patent Classification) — similar, used in international filings.

Look up the relevant CPC codes for your invention at the USPTO or EPO website. Searching by classification finds patents that may use completely different words to describe the same thing — which keyword searches miss.

Also brainstorm synonyms, related terms, and alternative phrasings. An inventor in Japan and an inventor in Germany may have described the same mechanism using entirely different vocabulary.

Step 3: Search Patent Databases

Google Patents (patents.google.com) The most accessible starting point. Covers patents and applications from over 100 countries, searchable in English (with AI-powered translation of foreign-language documents). Allows keyword, classification, and citation-based search.

USPTO Patent Public Search (ppubs.uspto.gov) Comprehensive coverage of all US granted patents and published applications. Supports sophisticated Boolean searches. (Replaces the legacy PatFT/AppFT databases, which USPTO retired in 2022.)

Espacenet (worldwide.espacenet.com) The EPO's free database, covering 150 million patent documents from over 100 countries. Particularly strong for European and international filings.

CNIPA (pss-system.cnipa.gov.cn) China's patent database. China now files more patents annually than any other country — do not neglect this database.

J-PlatPat (j-platpat.inpit.go.jp) Japan's patent database, with English search capability.

WIPO PATENTSCOPE (patentscope.wipo.int) PCT international applications and national patent collections from multiple countries.

Step 4: Search Non-Patent Literature

Limiting your search to patents is a common mistake. Non-patent literature (NPL) is a major source of prior art, particularly in rapidly evolving fields.

Google Scholar (scholar.google.com) — academic papers and conference proceedings across all disciplines.

PubMed (pubmed.ncbi.nlm.nih.gov) — biomedical and life sciences literature.

IEEE Xplore (ieeexplore.ieee.org) — engineering and electronics research.

arXiv (arxiv.org) — preprints in physics, mathematics, computer science, and related fields.

Web of Science / Scopus — comprehensive academic literature databases (subscription required, available at many university libraries).

Also search for commercial products in your field: manufacturer websites, industry trade publications, equipment catalogs, and trade show archives.

Step 5: Use Citation Analysis

Patents cite other patents. Once you find a relevant prior art document, look at:

• Backward citations — documents that this patent cites. These are patents and papers the examiner and applicant considered when filing.
• Forward citations — patents that cite this document. These are later inventions that built on it — potentially including competitors in your space.

Google Patents displays both forward and backward citations and allows you to navigate the citation network visually. This is one of the most efficient ways to map an entire technology landscape.

Step 6: Classify Your Findings

As you search, organize your findings into categories:

Highly relevant — describes your invention closely; may anticipate some or all of your claims. Review carefully and consider how your invention differs.

Moderately relevant — describes elements of your invention but not the combination. May be used by examiners in obviousness rejections.

Background art — establishes what the field looked like before your invention. Useful for writing the background section of your application.

Not relevant — same field, different technology. File for reference but no action needed.

Common Prior Art Search Mistakes

Searching only in English. Major innovations come from Japan, Germany, South Korea, China, and other non-English-speaking countries. Use Google Patents' translation capabilities and classification-based searching to capture foreign-language prior art.

Searching only granted patents. Published patent applications (18 months after filing) count as prior art even if they were never granted. Search both.

Focusing only on exact matches. Prior art does not need to be identical to anticipate your invention. An examiner can combine two prior art documents to argue your invention is obvious. Search broadly.

Stopping too early. Prior art searches feel complete before they are. Plan to spend at least 4–8 hours on a serious search, more for complex inventions.

Ignoring your own publications. If you have published papers, given conference talks, or posted online about your invention, find and document those disclosures. They are prior art against you in most of the world.

Professional Prior Art Searches

For commercially significant inventions, consider commissioning a professional prior art search from:

• A registered patent searcher
• A patent attorney or agent who includes searching in their services
• A specialist search firm

Professional searches typically cost $500–$2,000 and involve searchers with domain expertise who know how to navigate classification systems and non-patent literature in specialized fields. They also provide a written report that is useful documentation if your application is later challenged.

A professional search is not a substitute for your own initial search — you know your invention better than any searcher does — but it is a valuable complement, particularly before filing or making major investment decisions.

What to Do When You Find Relevant Prior Art

Finding relevant prior art is not cause for panic — it is valuable information. Your response depends on what you find:

Your exact invention exists: The idea is not patentable as described. Reconsider whether there is a genuinely novel variation, improvement, or combination worth pursuing.

A similar invention exists: Study the differences carefully. Your patent claims should focus on what distinguishes your invention from the prior art. A skilled patent attorney can help you craft claims that capture your genuine innovation while sidestepping the prior art.

Many related inventions exist: You are entering a crowded field. Not impossible to patent in, but your claims will need to be more precise. Consider whether strong patent protection is achievable and whether it is worth pursuing.

Very little exists: Either your invention is genuinely pioneering (great) or you have not searched well enough (search more).

Sources

1. Google Patents — Free patent search covering 100+ countries with AI-powered translation and citation analysis
2. Espacenet — EPO's free database of 150 million patent documents from over 100 countries
3. WIPO PATENTSCOPE — PCT international applications and national patent collections
4. USPTO - Manual of Patent Examining Procedure (MPEP) — Official USPTO guidance on prior art standards under 35 U.S.C. sections 102 and 103
5. 35 U.S.C. - Patent Law — Full text of US patent statutes including novelty and obviousness requirements