[ubernostrum]

The current precedent in the United States is that a patent holder cannot collect royalties past the expiration of the patent, and license agreements requiring payments beyond expiration are unenforceable.

This doctrine is being challenged at the Supreme Court currently, in Kimble v. Marvel:

http://www.scotusblog.com/case-files/cases/kimble-v-marvel-e...

[danray]

One wrinkle -- even under current case law, payments can still validly be collected for non-patent IP (most often trade secrets). Thus, the best practice from the licensor's is to delineate what portion of the royalty rate is attributable to the patent, the trade secret, the trademark, etc., and stop collecting each portion once the corresponding IP expires or is (finally, unappealably) held invalid.