Where there is a United States patent, there are also likely multiple foreign counterpart patents. Armed with a patent, a holder can then move to stop others from infringing on his invention, and more often than not, the defendant will argue that the United States patent is invalid, often citing foreign decisions and proceedings in support of that claim. Given the territorial nature of patents and the fact that countries have different requirements and standards for granting patents, United States courts have applied a presumption against giving preclusive effect to foreign patent decisions. The courts, however, have made clear that the presumption is a consequence of the differences in the patent law of different countries. The courts, therefore, seem willing and able to consider foreign decisions in United States proceedings. Given the ongoing efforts to harmonize United States patent laws with the patent laws of the rest of the world in order to meet the globalization of markets, whether the presumption against giving preclusive effect to foreign patent decisions will survive in its current form is very much an open question. Part II of this article offers a view of the traditional territorial nature of patents. Part III discusses how courts currently approach foreign patent decisions in United States proceedings and takes a look at the presumption against giving them preclusive effect. Part IV analyzes the harmonization efforts that have taken place and that are taking place in order to bring United States patent law in line with the rest of the world.