Congress has enacted laws requiring individuals and facilities to take measures to protect environmental quality and public health by limiting potentially harmful emissions and discharges, and remediating damage. Enforcement of federal pollution control laws in the United States occurs within a highly diverse, complex, and dynamic statutory framework and organizational setting. Multiple statutes address a number of environmental pollution issues, such as those associated with air emissions, water discharges, hazardous wastes, and toxic substances in commerce. Regulators and citizens take action to enforce regulatory requirements in a variety of ways to bring violators into compliance, to deter sources from violating the requirements, or to clean up contamination (which may have occurred prior to passage of the statutes). Implementation and enforcement provisions vary substantially from statute to statute, and are often driven by specific circumstances associated with a particular pollution concern. Given these many factors, it is difficult to generalize about environmental enforcement. This report focuses on enforcement of federal environmental pollution control requirements under the Clean Air Act (CAA); the Clean Water Act (CWA); the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA or Superfund); and other statutes for which EPA is the primary federal implementing agency. The report provides a brief synopsis of the statutory framework that serves as the basis for pollution control enforcement, including an overview of the key players responsible for correcting violations and maintaining compliance. Implementation and enforcement of pollution control laws are interdependent and carried out by a wide range of actors including federal, state, tribal, and local governments; the regulated entities themselves; the courts; interest groups; and the general public. Figure 1 , below, presents the array of local, state, tribal, and federal entities that constitutes the environmental pollution control enforcement/compliance framework and organizational setting. A diverse set of regulatory approaches and enforcement tools is applied to a sizeable universe of regulated entities by these multiple regulating authorities to ensure compliance. A general discussion of enforcement monitoring and response tools is included in this report, followed by a summary of recent fiscal year federal funding levels for enforcement activities. Discussion of available enforcement data sources, as well as tables illustrating examples of trends in enforcement activities, is presented in the two appendixes. While this report touches on many aspects of environmental enforcement, it does not describe every aspect and statute in detail. Rather, the report is intended to provide a broad perspective of environmental enforcement by highlighting key elements, and a general context for the range of related issues frequently debated. Information included in this report is derived from a variety of sources. These sources, including relevant subject-matter CRS reports providing in-depth discussion of specific topics and laws, are referenced throughout. Several themes reflecting congressional concerns over time since EPA was established in 1970 are reflected throughout the major sections of this report. Congress has conducted oversight, primarily in the form of hearings, on various aspects of the organizational infrastructure and operations designed to enforce pollution control statutes. These aspects of enforcement have also been the topic of investigations by the Government Accountability Office (GAO) and EPA's Office of Inspector General (EPA-OIG). The federal government's oversight of and coordination with states in implementing and enforcing federal pollution control laws have been of particular interest to Congress. The following sections briefly discuss some of the key issue areas. Since many, but not all, of the federal pollution control statutes authorize a substantial role for states, state autonomy versus the extent of federal oversight is often at the center of debate with regard to environmental enforcement. Not unexpectedly, given the "cooperative federalism" that is often used to characterize the federal, state, and tribal governments in the joint implementation and enforcement of pollution control requirements, relationships and interactions among these key enforcement players often have been less than harmonious. Disagreements involving environmental priorities and strategic approaches, and balancing the relative roles of compliance assistance with enforcement, contribute to the complexity and friction that come with enforcing national pollution control laws. Other contributing factors include the increasing number of statutory and related regulatory pollution control requirements (some with conflicting mandates) and the adequacy of the resources available for their implementation. The effects of variability among statutes, coupled with variability in federal and state interpretations and regulations, are often central to the debate. Some argue that this variability leads to too much inconsistency in enforcement actions from state to state, region to region, or between federal versus state actions. Others counter that this represents the flexibility and discretion intended by the statutes to address specific circumstances and pollution problems. A July 2007 GAO report found that progress had been made regarding federal oversight of state environmental enforcement programs, and that there had been improvements with regard to cooperative federal-state planning and priority setting. However, the GAO concluded that a greater effort was needed to achieve more consistency and effectiveness, and that these issues continue to need improvements. In a December 2011 report, the EPA OIG found that although "OECA had made efforts to improve state performance and consistency … state performance remains inconsistent across the country, providing unequal environmental benefits to the public and an unlevel playing field for regulated industries." The level of federal funding allocated to states and tribes to support effective enforcement of federal pollution control laws has also been a long-standing congressional concern. In March 2012, the Environmental Council of the States (ECOS) reported concerns among state environmental agencies with regard to the extent of reductions in federal funding for state environmental protection activities. In a 2008 study, ECOS reported that during 2005-2008 states expected spending to implement federal environmental laws to double while federal appropriations declined. Subsequently, ECOS reported that although federal funding for enforcement allocated to states increased marginally from FY2009 to FY2010, overall, reductions in state budget revenue are impacting their ability to maintain viable environmental enforcement programs. In a September 2012 ECOS report, budget environmental agency data collected from 49 states indicated that total funding from the federal government decreased between FY2011 and FY2012, and further decreases were expected for FY2013. In 2007, GAO reported that, although funding overall for enforcement activities had increased somewhat, it generally had not kept pace with the increasing number of mandates and regulations, or with inflation. The federal enforcement funding and personnel, primarily within EPA and the Department of Justice (DOJ), to ensure effective enforcement of environmental statutes has also been a concern of both appropriations and authorizing committees in Congress. Recently, in addition to funding priorities among the various EPA programs and activities (including enforcement), several promulgated and pending EPA regulatory actions have been central to debates on EPA's appropriations. Some Members expressed concerns related to these actions during hearings of EPA's appropriations, and authorizing committees continued to address EPA regulatory actions through hearings and consideration of legislation during the 113 th Congress. During the FY2011 and FY2012 appropriations deliberations, several provisions were proposed, and a subset adopted, that restricted the use of funding for the development, implementation, and enforcement of certain regulatory actions that cut across the various environmental pollution control statutes' programs and initiatives. Although no separate FY2013 Interior, Environment, and Related Agencies bill that includes funding for EPA was passed in the House and Senate, and no bills were introduced for FY2014, these regulatory actions remained prominent during the debate. Many other aspects of pollution control enforcement have been the subject of debate, and highlighted in congressional hearings and legislation. Some additional areas of continued interest include whether there is a need for increased compliance monitoring and reporting by regulated entities; impacts of environmental enforcement and associated penalties/fines on federal facilities' budgets (most notably the Department of Defense, or DOD, and Department of Energy, or DOE); how best to measure the success and effectiveness of enforcement (e.g., using indicators such as quantified health and environmental benefits versus the number of actions or dollar value of penalties); whether penalties are strong enough to serve as a deterrent and maintain a level economic playing field, or too harsh and thus causing undue economic hardship; how to balance punishment and deterrence through litigation with compliance assistance, incentive approaches, self-auditing or correction, and voluntary compliance; the effect of pollutant trading programs on enforcement; and the level of funding required to effectively achieve desired benefits of enforcement. These issues result from disparate values and perspectives among stakeholders, but also from the factors that are the focus of this report: the statutory framework, those who work within this framework, and the tools and approaches that have been adopted for achieving compliance with pollution control laws. The discussion below, beginning with identification of the principal statutes and key players, followed by an overview of integrated systems of administrative and judicial enforcement, compliance assistance, and incentive tools, is intended to provide a macro-perspective of environmental enforcement infrastructure and operations. As Congress has enacted a number of environmental laws over time, as well as major amendments to these statutes, responsibilities of both the regulators and the regulated community have grown. Organizational structures of regulatory agencies have evolved in response to their expanding enforcement obligations. Regulators also must adapt to an evolving, integrated system of administrative and judicial enforcement, compliance assistance, and incentive tools (see discussion under " Enforcement Response and Compliance Tools ," later in this report). The 11 laws listed in Table 1 generally form the legal basis for the establishment and enforcement of federal pollution control requirements intended to protect human health and the environment. The discussion in this report focuses on these federal environmental laws for which the U.S. Environmental Protection Agency (EPA) is the primary federal implementing agency. Since EPA was created in 1970, Congress has legislated a considerable body of law and associated programs to protect human health and the environment from harm caused by pollution. Those federal statutes, intended to address a wide range of environmental issues, authorize a number of actions to enforce statutory and regulatory requirements. Enforcement of this diverse set of statutes is complicated by the range of requirements, which differ based on the specific environmental problem, the environmental media (e.g., air, water, land) affected, the scientific basis and understanding of public risks, the source(s) of the pollutants, and the availability of control technologies. Regulatory requirements range from health and ecologically based numeric standards, or technology-based performance requirements, to facility-level emission and discharge permit limits.