Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A serial killer is traditionally defined as an individual who has killed three or more people over a period of more than a month, with down time (a "cooling off period") between the murders, and whose motivation for killing is usually based on psychological gratification. The motives for serial murder often include anger, thrill, financial gain, attention seeking or a sexual element. The murders may have been attempted or completed in a similar fashion and the victims may have had something in common; for example, occupation, race, appearance, sex, or age group. Learn more about serial killers, their common characteristics and motives, as well as some of the most notorious serial murderers in history in the following pages.