The goal of this article is to inform law enforcement personnel, including prison guards, about the types of behaviors and activities that career criminals, prison gangs, and Security Threat Groups (STGs) engage in so as to reduce the threat posed by these three groups. The authors contend that experience is the best teacher when it comes to understanding career criminals, and in an effort to assist other law enforcement officers, the authors note several characteristics of these types of criminals. Next, the authors discuss STGs, which are defined by Federal prison officials as an identifiable collection of inmates who pose a hazard to the penal system. These STGs may form prison gangs that are potentially more powerful than street gangs. Prison gangs recruit members and associates from within the prison and also recruit street gangs to carry out assignments. Finally, the authors discuss the ways in which criminal justice professionals should assess and deal with STGs and prison gangs. Generally, the approach is to assess group behavior rather than focusing on individual members. Criminal justice professionals must always be on guard because prison gang members constantly scrutinize guards and officers for any signs of weakness. In conclusion, the authors assert that a focus on the activities of career criminals, prison gangs, and STGs will make a major impact on the reduction of the interrelated threat posed by these three groups.