First to file and first to invent
First to file and first to invent 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-to-file system on March 16, 2013 after the enactment of the America Invents Act.
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 were the only countries to use first-to-invent systems, but each switched to first-to-file in 1989, 1998, and 2013 respectively. 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.
- 2138.05 "Reduction to Practice" [R-5] - 2100 Patentability
- 715.07(a) Diligence - 700 Examination of Applications
- "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
- Article I and the First Inventor to File: Patent Reform or Doublespeak?, in IDEA—The Intellectual Property Law Review, Volume 50, Number 3 (2010)
- 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
- Compilation of all papers on FTF constitutionality published between 2001-2009
- 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.