Defenses to allegations of patent infringement fall into two broad groups, statutory defenses and equitable defenses.
In statutory defenses, proof of such allegation will result in invalidity of the patent because it does not meet the requirement of the statute to obtaining a patent or that the patent is not infringed. These defenses include the public use and on sale defenses of 35 USC 102; or the defense of obviousness under 35 USC 103; or the defense of enablement and best mode under 35 USC 112.
The other broad class of defenses, equitable defenses, result in the unenforceability of the patent due to fraud and inequitable conduct, or misuse or delay in filing suit resulting in laches or estoppel. Laches, in general, refers to the conduct of a party which has placed the other party in a position where his rights will be imperiled and his defenses embarrassed. Usually, it is based on delay, attended by or inducing a change of condition or relation. An estoppel arises when one is concluded and forbidden by law to speak against his own act or conduct. Estoppel by laches is a failure to do something which should be done or to claim or enforce a right at a proper time.
Generally speaking, a defendant will respond to a complaint by alleging through the answer or counterclaim every defense possible for fear that failure to allege a defense will be later prohibited, and at the time of filing the answer, the defendant will not have had sufficient time for adequate discovery on many of the defenses.
Enforcement of a patent against otherwise infringing conduct may be precluded by three major defenses: