Editors' Summary: CERCLA § 107(a) authorizes EPA and private-party plaintiffs to recover prejudgment interest on outstanding costs related to CERCLA's response actions. This interest can amount to millions of dollars, and may be the single largest cost item at a site. Despite this, surprisingly little attention has been paid to prejudgment interest and how it is calculated. The authors, whose practice includes reviewing government-sponsored prejudgment interest claims and preparing such claims in private cost recovery actions, provide an introduction to calculating prejudgment interest under CERCLA and the issues that can arise in making this calculation in government and private cost recovery actions. They first examine how prejudgment interest rates are determined and suggest that the appropriate rate for both government and private actions may be the risk-free return on U.S. Treasury bills in which Superfund monies are invested. They next examine the calculation of prejudgment interest and enumerate the variables that must be known before this calculation can be made. The authors evaluate EPA's proposed amendment to the cost recovery provisions of the NCP that would require EPA to provide the amounts and dates on which interest is calculated and the amounts and total interest charges for a site. They then discuss how the time period for the accrual of prejudgment interest is determined, and analyze EPA's proposed amendment to the NCP that would define one of two alternative dates that CERCLA specifies for the commencement of the accrual period. The authors argue that the amendment would make a legislative change by regulatory means and that conforming the amendment to the statutory requirement may encourage EPA to provide specific information on its cost claims as soon as possible. Finally, they provide sample calculations of prejudgment interest and conclude that the magnitude of these amounts makes it advisable for both plaintiffs and defendants to devote more attention to the calculation, documentation, and evaluation of such claims.