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For a patent I am looking at, I see it expired on May 2, 2012, but that the owner can continue to "assert the patent after it expires with a look back period of six years"

What does this mean?

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Patent damages may be limited in a variety of ways. 35 U.S.C. § 286 provides for a “running” period limiting recovery of damages to no more than six years before commencement of a cause of action for infringement.

35 U.S.C. § 286 Time limitation on damages

Except as otherwise provided by law, no recovery shall be had for any infringement committed more than six years prior to the filing of the complaint or counterclaim for infringement in the action.

This is not a time limit on bringing the infringement suit; it is a time limit on recovering damages. The public policy idea behind this limitation is to discourage patent holders from sitting on their rights while damages accrue.

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So the important figure is not 6 years from expiry, but 6 years from the decision to file a complaint, yes? –  Rory Alsop Nov 9 '12 at 0:10

You've probably heard of a statute of limitations in a criminal law context, well, the same applies in patent law.

Patents, like many causes of actions based in contracts, have a 6 year statute of limitations.

Now what does that mean.

If a patent is infringed while it is enforceable i.e. before it expires, the patent owner can sue the infringer up to 6 years after the patent expires. This is important in a situation where the patent owner may not become aware of the infringement until after the patent expires.

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