Several major criminological and medical journal databases were searched for this study. All citations documented up to April 30, 2004, were identified by using the following key words: serial murder, serial homicide, serial killer, multiple murder, and multiple homicide. A total of 131 studies were identified, with approximately 75 percent focusing on serial murder. Slightly over 20 percent of the studies examined mass murder, and 6.1 percent examined multiple murder. FBI and psychological profilers were among the most active contributors to this area of scientific inquiry; however, many disciplines were represented in the research, including psychology, psychiatry, law enforcement, and sociologists. The methodological sophistication in this field is weak, since very few studies used statistical analyses, and comparison groups have been virtually absent. Overall, the field is in its infancy in terms of providing reliable data, data analysis, and credible theories for testing. The key obstacle for research in this field in the foreseeable future is access to data and to subjects that can be used in research agendas. 4 tables, 1 figure, 39 references, and appended coding guide for the review