The Zodiac Killer is the pseudonym of an unidentified serial killer who operated in Northern California in the late 1960s. The Zodiac murdered five known victims in the San Francisco Bay Area between December 1968 and October 1969, operating in rural, urban and suburban settings. He targeted three young couples and a lone male cab driver. The case has been described as "arguably the most famous unsolved murder case in American history", and has become both a fixture of popular culture and a focus for efforts by amateur detectives.

A serial killer is a person who murders three or more people, with the killings taking place over a significant period of time between them. The serial killers' psychological gratification is the motivation for the killings, and many serial murders involve sexual contact with the victims at different points during the murder process. The Federal Bureau of Investigation (FBI) states that the motives of serial killers can include anger, thrill-seeking, financial gain, and attention seeking, and killings may be executed as such. The victims tend to have things in common such as, demographic profile, appearance, gender or race. The FBI will focus on particular patterns that the serial killers follow throughout their murders. They will then use on the patterns they find for key clues into finding the killer along with their motives. Although a serial killer is a distinct classification that differs from that of a mass murderer, spree killer, or contract killer, there are overlaps between them.