Assault is defined and four aspects of the definition -- that it is not defined by injury, that it ranges in severity, that it refers to unlawful behaviors, and that it is present in many crimes -- are discussed in further detail. Extrafamilial child assault was examined using data from two sources: the Comprehensive Homicide File (CHF) and the 1988 household survey component of the National Incidence Study of Missing, Abducted, Runaway, and Thrownaway Children (NISMART). Several overall conclusions emerged from an analysis of the prevalence figures. First, most children murdered each year in the U.S. are killed by persons unrelated to them. There appear to be sizable numbers of children, aged 12 or younger, who are assaulted by persons outside their families. When assault is defined generally, girls are more likely to be victims of nonfamilial assault than boys. The data support the conclusion that nonfamily assault will occur despite the level of child supervision. The lives of assaulted teenagers were marked by higher levels of parent-child tension than those of non- assaulted teenagers. Finally, victims of nonfamilial assault were more likely than the general population of children to be living in households in which an adult had been a victim of childhood trauma. 18 tables, 4 notes, and 36 references