Why are patent holders allowed to sue years after their patent was infringed upon?
cperciva: Why is this allowed to happen?IANAL, but in general if you can show that a patent holder deliberately delayed enforcing their patent, any award of damages will be reduced under the clean hands doctrine.That said, there are many cases where patent enforcement takes a long time for no fault of the patent holder.  If the patent holder is an individual who doesn't have big pockets, it might take a few years before he is aware that his patent is being infringed; he might spend a year trying to get the infringing company to negotiate a licensing deal; he might spend a year trying to find a law firm willing to take the case on contingency; and that law firm might spend a year trying to negotiate and/or collecting evidence before actually launching a lawsuit.  So even without any deliberate attempts to delay enforcement it's entirely possible for a patent to be infringed for five years or more before a lawsuit happens.