First to file and first to invent
First to file (FTF) and first to invent (FTI) are legal concepts that define who has the right to the grant of a patent for an invention. The first-to-file system is used in all countries, including the United States, which switched to a first-inventor-to-file (FITF) system on March 16, 2013 after the enactment of the America Invents Act. There is an important difference between the strict nature of the FTF under the EPO and the FITF system of the USPTO. The USPTO FITF system affords early disclosers some "grace" time before they need to file a patent, whereas the EPO does not recognise any grace period, so early disclosure under the FITF provisions is an absolute bar to later EPO patent.
First to file
In a first-to-file system, the right to the grant of a patent for a given invention lies with the first person to file a patent application for protection of that invention, regardless of the date of actual invention.
First to invent
Canada, the Philippines, and the United States had been among the only countries to use first-to-invent systems, but each switched to first-to-file in 1989, 1998, and 2013 respectively. The concept of a "grace" period, under which early disclosure does not prevent someone from later filing and obtaining a patent must be distinguished here from the FTI system. Germany and the UK had formerly had a concept of grace. Both FTI and grace systems afforded the early discloser protection against later filers. The FTI system allowed non-disclosers to overturn established parties, whereas the grace system only protects early disclosers. The US moved to a grace system on 16 March 2013, which has been termed "first-to-disclose" by some writers.
Invention in the U.S. is generally defined to comprise two steps: (1) conception of the invention and (2) reduction to practice of the invention. When an inventor conceives of an invention and diligently reduces the invention to practice (by filing a patent application, by practicing the invention, etc.), the inventor's date of invention will be the date of conception. Thus, provided an inventor is diligent in actually reducing an application to practice, he or she will be the first inventor and the inventor entitled to a patent, even if another files a patent application, constructively reducing the invention to practice, before the inventor.
However, the first applicant to file has the prima facie right to the grant of a patent. Should a second patent application be filed for the same invention, the second applicant can institute interference proceedings to determine who was the first inventor (as discussed in the preceding paragraph) and thereby who is entitled to the grant of a patent. Interference can be an expensive and time-consuming process.
Assume Tom conceives of a new mousetrap on January 1, 2006. Tom works diligently from January 1, 2006, to February 1, 2006, to prepare a patent application, and Tom files his patent application on February 1, 2006. Thus, Tom constructively reduced his invention to practice on February 1, 2006. Assume Jerry conceives of the same mousetrap on January 10, 2006, and diligently files a patent application on the new mousetrap on January 20, 2006. Under the first-to-invent system, Tom is entitled to the patent on the mousetrap, because he conceived the mousetrap before Jerry and still worked diligently to reduce it to practice by filing.
As a further extension of the example, assume Tweety conceived of the same mousetrap on December 31, 1990. Tweety never told anyone about the mousetrap and did not work on reducing the mousetrap to practice for many years due to financial reasons. Tweety finally actually reduced the mousetrap to practice on February 15, 2006. Because Tweety did not diligently work to reduce the invention to practice in the period before others' conception of the same invention, he is not entitled to a patent over Tom or Jerry.
However, if Tweety has published his idea before 2006, then this publication can be a basis to reject or invalidate Tom or Jerry's patent.
Canada's change to first-to-file
Canada changed from FTI to FTF in 1989. One study found that the "reforms had a small adverse effect on domestic-oriented industries and skewed the ownership structure of patented inventions towards large corporations, away from independent inventors and small businesses." 
The USA's change to first-inventor-to-file (FITF)
With the America Invents Act of 2011, which was signed by President Obama on September 16, 2011. The law switched U.S. right to the patent from the previous "first-to-invent" system to a "first-inventor-to-file" system for patent applications filed on or after March 16, 2013. Many legal scholars have commented that such a change would require a constitutional amendment. Article I, Section 8, Clause 8 of the US Constitution gives Congress the power to “promote the Progress of ... useful Arts, by securing for limited Times to ... Inventors the exclusive Right to their respective ... Discoveries.” These scholars argue that this clause specifically prohibits a first-inventor-to-file system because the term "inventor" refers to a person who has created something that has not existed before.
Under the first-to-invent system, when two people claim the same invention, the USPTO would institute an interference proceeding between them to review evidence of conception, reduction to practice and diligence.
The change has not been short of detractors. For example, the IEEE stated in its submission to the House Judiciary Committee, charged with the study of the Patent Reform Act of 2007, that "We believe that much of the legislation is a disincentive to inventiveness, and stifles new businesses and job growth by threatening the financial rewards available to innovators in U.S. industry. Passage of the current patent reform bill language would only serve to relax the very laws designed to protect American innovators and prevent infringement of their ideas."
- Zach Carter (June 11, 2011). "Patent Reform Refuses To Die, Congress Keeps Cashing In". The Huffington Post. Retrieved July 31, 2013.
- USPTO: "America Invents Act: Effective Dates"
- Zuhn: "USPTO Issues First-Inventor-to-File Examination Guidelines and Final Rule"
- Kravets: "First-To-File Patent Law Is Imminent, But What Will It Mean?"
- REPORT FROM THE EUROPEAN COMMISSION TO THE EUROPEAN PARLIAMENT AND EUROPEAN COUNCIL "An assessment of the implications for basic genetic engineering research of failure to publish, or late publication of, papers on subjects which could be patentable as required under Article 16(b) of Directive 98/44/EC on the legal protection of biotechnological inventions"
- 2138.05 "Reduction to Practice" [R-5] - 2100 Patentability
- 715.07(a) Diligence - 700 Examination of Applications
- Lo and Sutthiphisal: "Does it Matter Who Has the Right to Patent: First-to-Invent or First-to-File? Lessons from Canada," April 2009, NBER Working Paper No. w14926
- 16 Sept 2011 whitehouse.gov press release re signature of AIA
- Glenn and Nagle: "Article I and the First Inventor to File: Patent Reform or Doublespeak?", in IDEA—The Intellectual Property Law Review, Volume 50, Number 3 (2010)
- Simon: "The Patent Reform Act’s Proposed First-To-File Standard: Needed Reform or Constitutional Blunder?", in THE JOHN MARSHALL REVIEW OF INTELLECTUAL PROPERTY LAW, 2006
- Selective compilation of papers on FTF constitutionality published between 2001-2009 (9 papers)
- Meredith and Grzelak: "“Letter to House and Senate Leaders and Judiciary Committee Members Opposing Adoption of the Patent Reform Act of 2007 (S. 1145/H.R. 1908).” The Institute of Electrical and Electronics Engineers, Inc. – United States of America, 27 August 2007
- From First-to-Invent to First-to-File: The Canadian Experience, Robin Coster, American Intellectual Property Law Association, April 2002.
- First-to-file or First-to-invent?, Charles L. Gholz, Journal of the Patent and Trademark Office Society, 82 JPTOS 891, December 2000. Advocates first-to-file for the US.
- First to Invent vs. First to File, Inventors Assistance League. Advocates first-to-invent.
- 1992 Special Summary Report; The Great Debate; First-to-invent vs. First-to-file and the International Harmonization Treaty, Stephen Gnass/Inventors Voice. Advocates first-to-invent as more friendly to the individual inventor.