U. S. energy needs rest heavily on ship-based imports. Tankers bring 55 percent of the nation's crude oil supply, as well as liquefied gases and refined products like jet fuel. This supply chain is potentially vulnerable in many places here and abroad, as borne out by several successful overseas attacks on ships and facilities. This book addresses (1) the types of threats to tankers and the potential consequences of a successful attack, (2) measures taken to protect tankers and challenges federal agencies face in making these actions effective, and (3) plans in place for responding to a successful attack and potential challenges stakeholders face in responding. The review spans several foreign and domestic ports, and multiple steps to analyze data and gather opinions from agencies and stakeholders. This is an excerpted and edited version of a GAO Report.