Editors' Summary: In recent years, fewer and fewer parties liable for Superfund site cleanup have successfully challenged the costs of cleanup reflected in claims made against them by the government. The government's burden in CERCLA cost recovery is to establish the amount of its response costs by a preponderance of the evidence. In this Article, the authors, who are U.S. Justice Department lawyers, assert that the government has better prepared cleanup cost summaries, making them generally sufficient to obtain summary judgment for the United States with respect to the amount of response costs. Still, the high financial stakes involved in Superfund cleanup tempt many liable parties to challenge the government, and the authors argue that once liability for CERCLA costs has been established, defendants squander time and money flyspecking costs during settlement negotiations and litigation. The authors first detail the procedures by which proof of response costs, which include intramural and extramural costs, is assembled, and then review the treatment of such cost packages by the courts, noting that, to their knowledge, in no action has recovery been denied for EPA intramural or extramural costs because of inadequate documentation of expenses. The authors next analyze the inability of liable parties to challenge the recover-ability of proven costs. Examining the statutory language of CERCLA §§ 107 and 113, the authors note Congress' intent to limit challenges defendants may make in cost recovery actions under § 107, and find no contradiction between the two sections, concluding that § 113 provides the standard to be used in determining consistency of response costs with the NCP. The authors also examine the relevant cases and conclude that they provide the government favorable standards in proving costs and limiting judicial standards of review. They conclude that though the cost recovery provisions of CERCLA appear to be harsh and unfair, they seem no harsher than CERCLA's imposition of strict, joint, and several liability.