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Congress sought to attain full compliance with environmental statutes. It reasoned that multiple enforcers would provide more comprehensive and effective enforcement than one enforcer. Congress therefore empowered the Environmental Protection Agency (EPA), the states and private citizens as enforcers of the statutes. However, Congress worried that successive actions by multiple enforcers could bring disruption and conflict to enforcement litigation and remedies. It therefore included in the citizen suit provision of each statute a limited, three-element notice, delay, and bar preclusion device to manage successive citizens' enforcement against the violations already subject to government enforcement. The device generally bars citizens from enforcing if the government “has commenced and is diligently prosecuting” an action involving the same violations “to require compliance.” However, Congress recognized that its goal of achieving full compliance by multiple enforcers and its subordinate goal of avoiding disruption and conflict from successive enforcement are in conflict. Congress resolved that conflict by the balance it struck between them in the particular formulation of the preclusion device it used in each statute. Courts have considered nearly a dozen legal issues arising from the bar element in the preclusion device. While many of these issues have been heavily litigated, two have gone relatively unnoticed. First, what did Congress intend when it limited preclusive actions to actions the government “is diligently prosecuting”? Does use of the present tense mean that only continuing prosecutions bar citizen suits? The plain reading of the verb suggests so, although that reading greatly limits the operation of the preclusions. Second, what did Congress intend when it limited preclusive actions to those the government brought to require compliance? Does the focus on compliance mean that only actions to require defendants to cease and desist from violating the statutes bar citizen suits? Again, the plain reading of the phrase suggests so, although that reading also greatly limits the operation of the preclusions. Most courts interpreting the preclusions have used plain meaning and expressio unius5 approaches to interpret the provisions, thereby respecting the balance Congress established between its goal of achieving full compliance through the efforts of multiple enforcers and its desire to avoid disruption and conflict from successive enforcement. There is no reason to depart from that interpretation of the provisions for the two issues examined here. Indeed, the structure of the provisions and their legislative history supports that interpretation. The conclusion that the statutory preclusions bar citizen suits only in the face of ongoing government actions for compliance, however, may not always allow citizen suits to enforce in the face of concluded government actions that have not achieved or never sought compliance. Common law doctrines may supplement the statutory preclusions. Res judicata, abstention and other related common law doctrines are designed to promote finality and to prevent disruption and conflict from successive litigation. Courts have not yet determined the relationship between the statutory and common law preclusions. Moreover, they have applied the common law preclusions inconsistently and some of their applications may also shield violators from effective enforcement or even compliance with the law.