Scholars and courts have struggled to make sense of the Supreme Court’s decision in United States v. Klein, an intriguing but enigmatic opinion concerning the limits of Congress’s ability to interfere with cases pending before the federal courts. Klein is intriguing because its broad and emphatic language suggests significant limits on the power of Congress. Klein is enigmatic because the Court has never again struck down a statute because of Klein or even made clear what principle animates its result. In fact, despite reaffirming the existence of a principle based on Klein, the Court has repeatedly read it narrowly, suggesting that the principle it embodies has not been adequately articulated. This Article argues that Klein’s principle is a specific application of a robust constitutional tradition that restrains governmental self-dealing. A Klein principle restraining governmental self-dealing explains the Court’s Klein cases, situates the principle within constitutional theory and doctrine, and provides much-needed direction to lower courts wrestling with questions about legislative intrusions into judicial functions.