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 the majority of countries, with the notable exception of the United States, which operates a first to invent system.
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
In a first to invent system, the right to the grant of a patent for a given invention lies with the first person to make that invention (with certain exceptions defined by the applicable law). The USA is the only significant country utilised a first to invent system.
The detailed implementation of such a system is complicated, since the date of invention must be proven to substantiate a right to a patent. Under US law, the first to file prima facie has the right to the grant of a patent. Should a second patent application be filed for the same invention, the applicant for that patent can institute interference proceedings to determine who was the first inventor and thereby who is entitled to the grant of a patent. This can be an expensive and time-consuming process.
Example
In an imaginary world, two inventors are secretly working on the greatest sure-kill mousetrap. Tom, the diligent first inventor, invented it. But he did not file the patent application. He wanted to improve it so it could be mass produced by ACME.
Jerry, working independently from Tom, developed the same invention shortly after Tom's success. Jerry had no use for the invention, but he wanted to bar anyone else from obtaining a patent on it. He hired his fast running cousin to deliver the patent application too and became the first to file an application.
Both of them kept the invention as a trade secret up to this point.
If Tom notices Jerry's move in time, he may prevail in an interference proceeding filed in the USPTO. If Tom disclosed the invention to the public before Jerry's filing, he can claim prior art and use it to invalidate Jerry's patent (Tom would have, at this point, a year to file a patent application. If he failed to file within this one-year grace period, the invention would enter public ___domain). If he fails to notice Jerry's move before the above-mentioned deadline, he can do nearly nothing to Tom. If he uses the invention secretly himself, he is not allowed to raise so-called "prior use right" as a legal defense in a patent infringement lawsuit because the subject matter is not a business method patent. He can try to negotiate a license from Jerry or catch the mouse in another way not covered by Jerry's patent.
It is usually hopeless for a copycat to forge evidence of first invention to hijack an issued patent or publicized patent application.
Comparison
The debate as to which system is better is long-running and unlikely to reach a single conclusion. There are arguments for and against both systems.
The first to file system leads to procedural certainty as the filing date of an application can very rarely be challenged. In contrast, the first to invent system leads to uncertainty as the right to grant of a patent can be challenged by a second party and can only be finally determined by extensive consideration of the making of the invention.
It is said, however, that the first to file system favours large companies who can afford to rapidly file patent applications, thereby gaining an advantage over smaller companies who are slower to file due to cost restraints. The first to invent system is therefore said to be beneficial in encouraging the growth of smaller companies. A potential problem with this argument is that a smaller company, filing second, would have to rely on interference proceedings to claim their patent, which may be beyond their economic reach and they are therefore no better off.