Vol. 120 No. 4 This Article analyzes the Supreme Court’s 2004 decision in Sosa v. Alvarez-Machain against the backdrop of the post-Erie federal common law. The Article shows that, contrary to the assertion of some commentators, Sosa did not embrace the “modern position” that customary international law (CIL) has the status of self-executing federal common law to be applied by courts without any need for political branch authorization and, indeed, is best read as rejecting that position. Commentators who construe Sosa as embracing the modern position have confounded the automatic incorporation of CIL as domestic federal law in the absence of political branch authorization (that is, the modern position) with the entirely different issue of whether and to what extent a particular statute, the Alien Tort Statute (ATS), authorizes courts to apply CIL as domestic federal law. The Article also explains how CIL continues to be relevant to domestic federal common law despite Sosa’s rejection of the modern position. The fundamental flaw of the modern position is that it ignores the justifications for, and limitations on, post-Erie federal common law. As the Article shows, however, there are a number of contexts in addition to the ATS in which it is appropriate for courts to develop federal common law by reference to CIL, including certain jurisdictional contexts not amenable to state regulation (namely, admiralty and interstate disputes), as well as gap-filling and interpretation of foreign affairs statutes and treaties. The Article concludes by considering several areas of likely debate during the next decade concerning the domestic status of CIL: corporate aiding and abetting liability under the ATS, application of CIL to the war on terrorism, and the use of foreign and international materials in constitutional interpretation.