There are several potential defenses to a patent infringement claim that a defendant may raise, including:  Invalidity: The defendant may argue that the patent in question is invalid, either because it was not properly granted by the patent office, or because it does not meet the legal requirements for patentability.  Non-infringement: The defendant may argue that their product or process does not infringe on the patent, either because it operates differently or because the patent claims are too broad or vague.  Prior art: The defendant may argue that the invention claimed in the patent is not novel or non-obvious, and that similar inventions existed before the patent was filed.  Experimental use: The defendant may argue that they were using the patented invention for experimental purposes, in order to improve upon it or develop a competing product.  Estoppel: The defendant may argue that the patent holder made misleading statements or engaged in other behavior that should prevent them from enforcing the patent against the defendant.  License: The defendant may argue that they have a license or other permission to use the patented invention, either from the patent holder or from another party with the right to grant licenses.