Nullification Theory is the idea that states have the right to invalidate any federal law that they believe contradicts their own interests or the Constitution. This theory emerged during a time of growing tensions between federal and state authority, particularly regarding issues like tariffs and the rights of states versus the central government. It was prominently championed by figures like John C. Calhoun in the early 19th century and played a significant role in shaping political debates about states' rights and regional interests.