Since the enactment of the Clean Water Act (CWA or Act) 28 years ago, the federal courts have been called upon to sort out the respective roles of the federal and state governments in connection with numerous aspects of the statute's implementation and enforcement. Congress has superimposed an additional layer of complexity on the CWA experiment in creative federalism—the citizen suit provision. Through this provision, Congress authorized affected citizens to sue alleged violators, but circumscribed their right to do so when the federal or a state government is taking or has taken enforcement action of its own.

Not surprisingly, courts have had to grapple with numerous issues concerning when citizen suits are precluded by government enforcement. In general, the courts have resolved these issues based upon their underlying views as to the relative importance of citizen suits in the statutory enforcement scheme. Courts emphasizing the primacy of government enforcement discretion and the secondary role of citizen suits construe the preclusion provisions more aggressively. Courts placing a higher value on the role of citizen suits as a check on potentially lax government enforcement construe the preclusion provisions more narrowly. This Article examines the more important and frequently litigated issues concerning the scope of the Act's citizen suit preclusion provision. These are the major issues practitioners should keep in mind when advising clients about the risks of a citizen suit and when evaluating how to manage the defense of a citizen suit to optimize the outcome for the client.