Like most criminal investigators, active and retired, I owe the informants and cooperating witnesses I recruited and operated over the years for many, if not most, of the investigative successes I enjoyed. Some threatened the outcome of costly investigations. Each informant I worked with presented unique challenges. Informants often face incredible risks for their handlers. Some try to take advantage of their relationship with law enforcement in a system ripe with opportunities for abuse. My experience with informants has been mixed. An informant saved my life during an undercover heroin operation. One of the informants I utilized was murdered in a Miami alley when she was suspected of cooperating with the police. I’ve had to deactivate a few informants for misconduct. The cooperating witness in a smuggling investigation I participated in had to enter the U.S. Marshals Witness Security Program because of death threats, never to be heard from again. I’ve helped send more than one informant to prison. My experience with informants has proven invaluable in my later career as a criminal attorney and international criminal law advisor. Regardless, this book could not have been written was it not for the experience of working side by side in highly charged situations with an odd mix of felons, prostitutes, and con artists. Unfortunately, they all must go unnamed. I am sure they would want it that way. Informants and cooperating witnesses have occupied commanding roles in some of the highest stakes domestic and international criminal investigations of the twentieth and twenty-first centuries. Likewise, so-called routine criminal cases generated in whole or in part through information received from informants and cooperating witnesses fill the dockets of courtrooms across the United States. While confidential sources are an inextricable thread of the fabric of American law enforcement, their use does not come without substantial risk and controversy. There is no shortage of criticism regarding a police practice many regard as nothing less than dealing with the devil.