Adolescence, as a stage of human development, has always been regarded as a time in which risk-taking behaviors are common. The likelihood that an individual will engage in aggressive or violent behaviors during the middle school and high school years is illustrated when we take a broad view of data that has emerged about adolescent aggressive and violent behaviors over the past fifteen years. Currently for adolescents the ages 14 to 18 is a period when 42% of boys and 28% of girls in the United States acknowledge having had a physical fight in the past year (USDHHS, 2006a). Death as a result of homicide consistently ranks second among the causes of death among 15 to 24 year olds, the highest ranking for homicide of any age group across the lifespan (NCHS, 2004). Crime surveys have consistently shown that 12 to 19 year olds have the highest rates of victimization by violent crime of any age group (Snyder, 2004; Snyder & Sickmund, 1999). Exploration of variation over time in prevalence rates for aggressive and violent behaviors during adolescence, and variations with grade, gender, and race, will tell us what is happening in the real world and who is at greatest risk. This macro-examination of relatively fixed and key markers will be followed by narrower focus on the early development of aggression and violence (in Chapter 2), and by discussion of more malleable personality and situational risk factors (in Chapters 3 and 4).