Studies have shown that 25 percent of international child sex tourists come from the United States. For American law enforcement agencies, the typical CST investigation involves a U.S. citizen or resident traveling abroad to engage in commercial sex with a child. Much of the evidence needed for prosecution remains in another country with laws, societal attitudes, and law enforcement responsiveness different from those of the United States. Limited resources, lack of specialized training, and language barriers add to the difficulties of these transnational investigations. The Federal Protect Act of 2003, however, has eliminated the requirement that investigators provide evidence of the offender's intent to engage in commercial sex with a child prior to departing from the United States. This clears the way for aggressive prosecution in cases that previously would have proven impossible to pursue in U.S. courts. This article describes both reactive and proactive techniques used by the Federal Bureau of Investigation's (FBI's) Miami office to pursue child sex tourists in the United States. Reactive investigations involve cases in which the victimization has occurred or is imminent. They generally begin with a citizen's complaint or a victim's statement. Proactive investigations, on the other hand, use sophisticated investigative techniques, such as undercover operations, to identify offenders and prevent the abuse of potential victims. This article also describes the characteristics of 13 individuals arrested as a result of the FBI's undercover operation in Miami. The FBI's first international undercover operation designed to combat CST demonstrated the importance of joint, proactive law enforcement initiatives that cross national boundaries.