Growth in North American crude oil and natural gas production has resulted in efforts to expand the domestic oil and natural gas pipeline network. Pipelines can be a cost-effective and comparatively safe means of hydrocarbon transport. Pipelines and their routes also can be controversial locally, regionally, or nationally. While many new or expanded pipelines have been proposed and completed in recent years, other proposed pipelines have been unable to satisfy all the requirements to obtain federal authorizations, have faced barriers among state regulatory agencies, and have been the subject of litigation. Congress has a long-standing interest in pipeline development and the regulation of pipelines because of the critical role of pipelines in the domestic energy markets. Failure to construct pipelines may result in various potential effects, such as greater reliance on road or rail transport and constraints on getting oil and natural gas resources to refineries, power plants, and other consumers—all of which can create economic inefficiencies. Local interests along proposed pipeline routes regularly have raised concerns about the local environmental and other impacts of pipeline construction; they also have expressed concerns about the risks during pipeline operations of accidents that could affect the environment and human health. In recent years, stakeholders interested in reducing U.S. reliance on fossil fuels or promoting other energy sources and energy conservation have brought greater public attention both to oil and natural gas pipelines and to federal and state decisions related to new pipelines in particular. Some of these pipeline opponents argue that pipelines may facilitate the use of oil and natural gas, thereby indirectly contributing to the human health, welfare, and environmental effects of oil and natural gas production and use. Pipeline proponents argue that the impacts of development and use of natural gas may be less than impacts associated with other energy sources and that both oil and natural gas pipelines contribute to the nation's energy security and provide broad benefits to the U.S. economy. This report discusses how the U.S. Army Corps of Engineers (Corps) administers its responsibilities related to certain pipeline segments that require Corps authorizations. The Corps is one of many federal agencies with responsibilities that may relate to oil and natural gas pipelines. Interest in federal approvals and requirements for pipelines relates to a broader policy issue before Congress: What is the appropriate federal role in protecting the environment, public health, national economy, and domestic energy security when reviewing and authorizing activities associated with private development of energy-sector or other infrastructure? The role of the Corps in authorizing oil and natural gas pipelines is limited to (1) the agency's regulatory authorities for authorizing activities affecting regulated water bodies and wetlands and activities that alter or obstruct navigable waters and (2) its civil works authorities, including allowing a pipeline to cross a Corps water resource project and related lands. Because most pipelines cross regulated water bodies, the Corps must authorize the activities at those crossings before construction at those locations can commence. Because the Corps has no broad authority related to pipelines along their entire domestic routes, the agency's analyses to support its authorization decisions are scoped to focus on the application of Corps authorities to specific pipeline segments. Controversy regarding the Dakota Access Pipeline (DAPL), a crude oil pipeline from North Dakota to Illinois, focused attention on the Corps' authorizations and approvals. This attention to the Corps may continue, given the renewed efforts for the international Keystone XL, proposed domestic pipelines, and related litigation. This report provides an overview of the agency's role with respect to oil and natural gas pipelines. Because the Corps' pipeline-related decisions can be controversial, the report also introduces some of the issues raised. After an overview of the Corps and the broader federal role in approving private oil and natural gas pipelines, the remainder of the report discusses how the Corps' regulatory responsibilities apply where pipelines cross water bodies and wetlands (i.e., Corps regulatory program); how the Corps' civil works authorities may apply if a pipeline may alter or affect a Corps water resource project or may cross Corps-managed land (i.e., Corps civil works program); how the Corps complies with various federal statutes as it arrives at pipeline-related decisions, including the environmental documentation requirements of the National Environmental Policy Act (NEPA) and the historic preservation requirements of the National Historic Preservation Act (NHPA); and how the agency's role in pipelines raises or illustrates various policy issues for decisionmakers. Siting, construction, and operation of oil and natural gas pipelines may be subject to a number of local, state, tribal, and federal requirements. Whether the entire pipeline will require federal agency authorization generally depends on the commodity the pipeline would transport and whether it would cross state lines. Specifically, the siting, construction, and operation of interstate natural gas pipelines must be authorized by the Federal Energy Regulatory Commission (FERC), in accordance with requirements established under Section 7(c) of the Natural Gas Act of 1938; in contrast, domestic oil pipelines and intrastate natural gas pipelines are not subject to a similar federal authorization for their entire routes. Federal pipeline safety standards also could affect pipeline siting, construction, operation, and maintenance. The Department of Transportation's Pipeline and Hazardous Materials Safety Administration (PHMSA) is the principal federal agency charged with ensuring the safe interstate movement of natural gas and hazardous liquids, including crude oil. Congress tasked the Department of Transportation, working through PHMSA, with regulating key aspects of interstate pipeline safety, including design, construction, operation and maintenance, and spill response planning. For more information on the Department of Transportation's safety program, see CRS Report R44201, DOT's Federal Pipeline Safety Program: Background and Key Issues for Congress , by [author name scrubbed], and see the box later in this report titled "PHMSA Pipeline Safety Regulation and Corps Permit Conditions Related to Safety." Specific aspects or activities associated with pipelines may be subject to federal agency approval. For example, separate federal agency authorization (in the form of a permit or other approval) may be required if the proposed pipeline, in whole or in part, would cross federal land or potentially would affect an environmental, natural, or cultural resource protected under federal law. As discussed below, the involvement of the Corps in pipelines generally is limited to those pipeline segments that would cross Corps-managed federal land (hereinafter referred to as Corps land ) or a Corps-managed federal flood easement on nonfederal land and to those pipeline activities that may affect or cross waters and wetlands regulated by the agency. Although the Corps has no authority to approve pipelines, certain pipeline segments and their construction may require Corps authorizations. Under the agency's regulatory program, the Corps is responsible for authorizing activities that could affect federally regulated waters and wetlands. Under its civil works program, the agency is responsible for approving activities that cross or affect Corps lands and projects. A pipeline developer must have authorization from the Corps regulatory program (sometimes referred to as its permit program) for any pipeline segment affecting or crossing Corps jurisdictional water, in accordance with Section 404 of the Clean Water Act, under which the Corps regulates the discharge of dredged or fill material into waters of the United States, including wetlands; and Section 10 of the Rivers and Harbors Act of 1899, under which the Corps regulates structures and/or work in or affecting the course, condition, or capacity of navigable waters. Because pipelines of any significant length will cross or otherwise may affect U.S. waters somewhere along their routes, pipeline developers are routinely required to have authorization from the Corps under Section 404 of the Clean Water Act (hereinafter Section 404) and/or under Section 10 of the Rivers and Harbors Act (hereinafter Section 10). The Corps regulates all activities affecting regulated waters; for pipelines, the Corps regulates activities whether the pipeline is interstate or intrastate and whether the pipeline is transporting oil or natural gas. A pipeline developer may need certain approvals from the civil works program of the Corps if the proposed pipeline could affect a Corps water resource project or Corps land and real estate interests. Corps land and other real estate interests (e.g., private lands with a federal flood or other flowage easement) typically were acquired as part of a water resource project. For pipelines to cross below, on, or above Corps land and non-Corps land with a Corps real estate interest, the agency generally will decide on granting both a Corps permission to alter a Corps civil works project; and a Corps easement to cross Corps land or a consent to cross non-Corps land with a Corps real estate interest. Figure 1 illustrates pipeline segments over which the Corps may have some role, specifically segments that would cross Section 404 Corps-regulated waters and wetlands or Section 10 Corps-regulated waters (and therefore would involve a role for the agency's regulatory program) and Corps land (and therefore would involve a role for the agency's civil works program). Figure 1 also illustrates FERC involvement in authorizing the siting and construction of interstate natural gas pipelines; no federal agency has a similar role for domestic interstate or intrastate oil pipelines or for intrastate natural gas pipelines. Stakeholders with an interest in pipeline construction sometimes have focused on the role of the Corps in pipeline-related approvals when no other federal agency has authority to approve the entire pipeline, which primarily is the case for domestic pipelines transporting crude oil. In the case of the DAPL to transport crude oil, the pipeline route selected crossed Corps-regulated waters and Corps land at Corps water resource projects. For more on the DAPL, see the box "Why Was the Corps the Main Federal Agency Involved in the DAPL Debate?" The Corps is responsible for authorizing activities that may affect waters over which it has jurisdiction under Section 404 or Section 10. The agency performs this work as part of its regulatory program. The types of projects that may cause impacts to regulated waters vary widely, including navigation, coal mining, riverbank stabilization, and transportation projects. One category of activities is a water or wetland crossing by a utility line, which includes oil and natural gas pipelines.