Contact with the justice system is a part of adolescence for approximately 1 million youths each year. The Federal Bureau of Investigation (FBI) has reported that people younger than age 18 comprised approximately 10% of all arrests, or more than 1 million charged offenses, in 2012; 3% of arrests were of youths younger than age 15 (FBI, 2013). Youths younger than age 18 were involved in a significant number of arrests for crimes the FBI considers serious, including murder, rape, robbery, and aggravated assault (see Figure 7.1). Male youths younger than age 18 comprised 10.4% (n = 722,855), and female youths younger than age 18 comprised 12% (n = 297,479) of all male and female arrests in 2012, respectively (FBI, 2013). White youths represented the majority of people younger than age 18 arrested for forcible rape, aggravated assault, burglary, larceny or theft, motor vehicle theft, and arson (see Figure 7.2).These often-cited numbers give an overview of the scope of the problem and emphasize the importance of effective policy and practice responses, but they mask an immense amount of detail. Who are these offenders? Are they first-time offenders or experienced offenders? Will they all continue to commit crime? If so, will their offending escalate? These are important questions that are not addressed by descriptive information about single incidents. In this chapter, we examine some aspects of these questions in greater detail to unpack those numbers and learn more about patterns of offending, particularly among serious offenders. First, we examine how key terms for research and policy on serious offenders and offending patterns are defined and used. Next, we