Tensions are growing in the Taiwan Strait. Chinese warplanes have violated Taiwan’s air defense identification zone in record numbers, prompting fears of an invasion. 2021 was the first year in which a potential crisis over Taiwan rose to the level of a “Tier 1 risk” in the Council on Foreign Relations’ Preventive Priorities Survey, which is an annual survey of American foreign policy experts. The United States faces a decision about what it can do to help prevent crossstrait tensions from escalating into war. A vital question is whether, and how much, the United States should change its “One-China policy.” Beginning with the Trump administration and continuing with the Biden administration, the United States has bolstered its support for Taiwan and become more assertive in resisting Beijing’s claims of sovereignty over the self-governing democracy. Although U.S. officials stress that the United States continues to adhere to the One-China policy, there is growing concern, as expressed by Daniel Russel of the Asia Society Policy Institute, that the United States is “edging closer and closer to the line that separates unofficial relations with official relations, which, in effect, could hollow out America’s One-China policy.” How much flexibility is built into the One-China policy, and what limits does the policy impose on what the United States can do to support Taiwan? To answer those questions, this policy brief explains what the One-China policy is and how it can exhibit both continuity and change. Analysis of the One-China policy often focuses on the Three U.S.-China Joint Communiqués, the Taiwan Relations Act, and the Six Assurances, which can create the impression that the One-China policy has not changed since those texts were formulated in the 1970s and 1980s. In fact, the One-China policy has been revised, such as in the Taiwan Policy Review during the Clinton administration. The One-China policy is neither set in stone nor completely fluid, and this policy brief identifies which elements are fixed and which elements are variable. It explains the One-China policy at three levels: the fundamental position, the doctrinal statements, and the practices and conventions. These levels of policy range from the most abstract to the most concrete, but each is logically consistent with the others.