Wilful infringement is when someone knows that an idea is patented, and proceeds to use that idea without obtaining permission.
This concept may have other names, or may not exist, in various jurisdictions, but it exists in the European Union, and "the willful infringement doctrine" exists in the USA. It may also be called a "bad faith infringement".
 In the USA
The question of whether an infringement is willful is usually important regarding the fine to be paid by a defendant who loses. In the USA, a court can increase the damages to up to three times the original amount in the case of willful infringement, and the judge can order the defendant to pay the plaintiff's attorney's fees.
In the USA, the 2007 In re Seagate case is seen as having made it more difficult to have an infringement judged "willful".
When a patent holder sends a notification letter (cost: 44 cents for a stamp) to a software developer, the alleged infringement becomes an alleged willful infringement. The software developer can undo this by getting a certificate of non-violation from a lawyer, which patent attorney Dan Ravicher estimates to cost $40,000.
 Cases of wilful infringement
- I4i v. Microsoft (2009, USA)
- Stac v. Microsoft (1993, USA)
- Calculating infringement damages in the USA
- What to do if worried by a software patent
- Using Reexaminations to Avoid Willfulness Damages, Sep 2009 Patently-o
- Patent Defence and Free Software, March 2010, Andrew Tridgell advises against exagerating the issue of willful infringement
- http://www.pubpat.org/assets/files/ftisapresentation/RavicherFTISAPresentation.mp3 (time:13:45)
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