Abstract: For two decades, courts in the Ninth Circuit enforced the so-called Federal Defendant Rule, under which intervention as of right was prohibited in cases brought under the National Environmental Policy Act (NEPA). The Ninth Circuit eventually abandoned this rule in its 2011 en banc decision in Wilderness Society v. U.S. Forest Service. This Article traces the history of the Federal Defendant Rule, showing how it evolved through a common law-like process from a fact specific decision in one case to a bright-line rule. It also explains how, despite the Rule’s apparent clarity, it produced confusion in the district courts of the Ninth Circuit, leading to a series of inconsistent decisions. The Article concludes that the Ninth Circuit was right to reject the Rule and uses the history of the Rule to draw more general lessons about the processes through which judicial doctrines emerge, evolve, and are abandoned.