Between overt diplomacy and armed conflict is the more subtle game of policy influence. Nation-states and non-state actors both use a variety of means to encourage preferred decisions by the U.S. government. Such activities to exert influence may create more entrenched challenges to security than some of the front-page problems of terrorism, espionage, and crime. The more effectively a foreign actor can subvert policy making at the outset, the fewer resources this foreign actor will need to expend on attacking the status quo later. Additionally, foreign governments and other actors may attempt to sow dissension and distrust of Washington among elements in the United States, in order to constrain the U.S. government's ability to act in a decisive manner. In addition to identifying vulnerabilities, the U.S. intelligence community must also deal with the surreptitious aspects of influence (e.g. unregistered foreign agents, intelligence officers engaged in collection to further understand targets' vulnerabilities, etc.). Hostile countries such as Russia and China have longstanding histories, in some instances reaching to the early days of the Cold War, of pursuing contacts with at people the highest levels of government, prominent academic voices well-positioned to nudge decision making, as well as grassroots ideological, activist, and cultural movements that can put an American face on a foreign agenda. Even friendly governments are not immune from employing similar tactics. The litany of historical examples directed at the United States, as well as continued, high-profile examples at home (e.g. the New York Times' identification of foreign funding to ostensibly independent think-tanks) and abroad (e.g. Russian financial assistance to far-right parties in Europe) point to a persistent problem about which too little has been written. Moving beyond sensationalistic accounts of foreign influence over U.S. policy making, this books addresses a growing issue in U.S. security and intelligence.