Robert Ressler is a former FBI agent and author. He coined the term serial killer and played a significant role in the psychological profiling of violent offenders in the 1970s.

He served in the US Army before joining the FBI in 1970. Ressler was recruited into the Behavioural Science Unit that deals with drawing up psychological profiles of violent offenders who typically select victims at random, such as rapists and serial killers. He came up with the term "serial killer" both from the fact that the repeated nature of homicides committed by such individuals reminded him of television serials he watched as a child, and from the term "crimes in series" as used by British detectives.

In the early 1980s, Ressler helped to organize the interviews of thirty-six incarcerated serial killers in order to find parallels between such criminal's backgrounds and motives. He was also instrumental in setting up Vi-CAP (Violent Criminal Apprehension Program). This consists of a centralized computer database of information on unsolved homicides. Information is gathered from local police forces and cross-referenced with other unsolved killings across the United States. Working on the basis that most serial killers claim similar victims with a standard method (modus operandi) it hopes to spot early on when a killer is carrying out crimes in different jurisdictions. This was primarily a response to the appearance of nomadic killers who committed crimes in different areas. So long as the killer kept on the move, the police forces in each state would be unaware that there were multiple victims and would just be investigating a single homicide each, unaware that other police forces had similar crimes. Vi-CAP would help individual police forces determine if they were hunting for the same perpetrator so that they could share and correlate information with one another, increasing their chances of identifying a suspect.

He worked on many cases of serial homicide such as Jeffrey Dahmer and Richard Chase.

Ressler retired from the FBI in 1990 and is the author of a number of books about serial murder. He is active giving lectures to students and police forces on the subject of criminology, and in 1993 was brought in to London to assist in the investigation into the murders committed by Colin Ireland.