Abstract: It is often said that the legal touchstone of agency independence is whether agency heads are removable at will or only for cause. Yet this condition is neither necessary nor sufficient for operational independence. Many important agencies whose heads lack for-cause tenure protection are conventionally treated as independent, while other agencies whose heads enjoy for-cause tenure protection are by all accounts thoroughly dependent upon organized interest groups, the White House, or legislators and legislative committees. This Article argues that the crucial role is played by what Commonwealth lawyers call "conventions." Agencies that lack for-cause tenure yet enjoy operative independence are protected by unwritten conventions that constrain political actors from attempting to remove their members or to direct their exercise of discretion. Such conventions reflect norms within relevant legal and political communities that impose sanctions for violations of agency independence or create beliefs or internalized moral strictures protecting independence. Conversely, where agencies enjoy statutory independence yet lack operative independence, the interaction among relevant political actors has failed to generate protective conventions. The lens of convention helps resolve several puzzles about the behavior of Presidents, legislators, judges, and others with respect to agency independence-including the Supreme Court's puzzling treatment of SEC independence in Free Enterprise Fund v. PCAOB. By acknowledging the conventional character of agency independence, U.S. courts can incorporate ideas from the courts of Commonwealth legal systems that harmonize conventions with written rules of law. This Article's principal suggestion is that U.S. courts should adopt the leading Commonwealth approach, according to which judges may indirectly "recognize" conventions and incorporate them into their interpretation of written law, but not directly enforce conventions as freestanding obligations.