The two main sources of data on homicide rates are coroners' reports and law enforcement statistics; both are affected by many factors. Nevertheless, the available data show a relatively steep increase between 1900 and 1933, an overall decline through 1964, an increase until 1980, and subsequent decreases. The last 20 years are the most homicidally violent decades in this century in the United States. Although data on victim-offender relationships are incomplete, killings of young men by other young men of similar age and race and who are acquainted is a persistent form of homicide. When homicide rates increase, homicides involving strangers or economic relationships increase. Structural, cultural, interactionist, and psychosocial factors may predispose individuals to kill. These factors vary for family homicides and for homicides related to robbery, however. Further research on the factors involved is needed. Figure, table, and reference notes.