Patent infringement litigation is not limited to determining whether infringement has occurred. Issues concerning patent validity, the existence of infringement, the enforceability of patent rights, and the applicability of statutory exceptions may also arise. These distinct issues are, however, sometimes examined under the single heading of “defenses,” despite their different legal bases and judicial effects. This study identifies appropriate criteria for classifying defenses and determining their legal effects, focusing on Iranian law and comparing it with United States law. The research adopts a descriptive, analytical, and comparative methodology based on legislation, judicial decisions, and scholarly literature. The findings indicate that the designation or legal source of a defense alone does not provide an adequate basis for classification. Rather, the subject matter challenged and the legal effect of accepting the defense should constitute the principal criteria. Accordingly, defenses may be classified into four categories: defenses concerning patent validity, non-infringement defenses, defenses concerning the exercise or enforceability of patent rights, and statutory exceptions. Iranian law contains elements of all four categories across different provisions, but their legal effects have not been organized within a coherent framework. United States law likewise demonstrates the importance of distinguishing invalidity, non-infringement, unenforceability, patent exhaustion, and statutory exceptions in determining the scope of judicial decisions. The study proposes that each defense be assessed according to its subject matter, legal basis, scope of effect, and judicial consequence.