behaviors set the stage for adult behavior in terms of use of substances (both legal and illegal) and dietary habits and can have an impact on the development and outcome of medical illnesses, such as cardiovascular disease, diabetes, obesity, osteoporosis, and HIV/AIDS.

The past two decades of research have revealed that many mental disorders are relatively common in adolescence. Details of epidemiological studies of mental disorders in adolescents are presented in each of the disorder-focused chapters in this volume. Some of the more striking examples are the following:

What is especially alarming is that the prevalence of some of these disorders has been on the rise over each successive generation. Certain changes over time in the nature of adolescence, and the environments that adolescents find themselves in, may be responsible for these observed increases in the prevalence of psychopathology in adolescence. A major factor is that adolescence itself is now more extended. Puberty has been occurring progressively earlier, particularly in developed countries such as the United States (Parent et al., 2003). At the other end, full-time work and marriage now occur later in life. Thus, if adolescence is defined in terms of the onset of puberty, the total time spent in adolescence is now longer than in the past, and if its upper end is defined as the end of formal schooling, the total time is now much longer. Access to and availability of potentially harmful environments and substances have increased. For example, many types of abusable drugs can now be ordered through the mail via the Internet (For man, 2003; National Drug Intelligence Center, 2002). Compounding the potentially negative consequences of harmful environments is the increasing behavioral independence of adolescents in association with less parental or even adult influence.

There are many unanswered questions about the ways in which the interplay between biology and environment lead to the alarming numbers of adolescents we now see afflicted with mental illness and why this seems to have worsened in recent years. However, what is clear is the need to make adolescent mental health a major public health priority. A decade ago, early childhood moved into the spotlight and became a major health priority, but from the point of view of mental health, adolescence may be the more critical transitional period given its neurobiological and behavioral plasticity. It is, moreover, likely the optimal time for prevention and treatment of psychopathology, and for the promotion of mental health and positive emotional and behavioral functioning. By increasing our knowledge of the causes, treatment, and prevention of mental disorders that begin in adolescence, we will help reduce the suffering and impairments associated with these disorders and reduce overall health care utilization. Furthermore, progress in adolescent mental health could prevent mental disorders in adulthood that have onset in adolescence and modify the prevalence or course of medical illnesses in adulthood that are related in part to adolescent behaviors or mental disorders.

For the current volume, we have chosen to concentrate on mood disorders, anxiety disorders, eating disorders, suicide, substance use disorders, and schizophrenia. These disorders represent the major mental disorders or public health issues among adolescents, with the exception that conduct disorder and attention-deficit hyperactivity disorder, two important disorders of adolescence, are not represented in the current volume. This decision was made because these disorders have clearer roots in childhood and they were extensively covered in the parallel

book, A Guide to Treatments That Work (Nathan & Gorman, 2002), which focused primarily on adults.

Adolescence is a distinct developmental period characterized by significant changes in hormones, brain and physical development, emotions, cognition, behavior, and interpersonal relationships. It has been defined as beginning with the onset of sexual maturation (puberty) and ending with the achievement of adult roles and responsibilities (Dahl, 2004). As mentioned previously, in terms of chronological age, the range for adolescence is broadly inclusive, roughly 10 to 22 years of age. For a number of reasons, this range is only a guide. First, there are wide individual differences in development. The onset of puberty, along with its associated hormonal and physical changes, occurs significantly earlier for some youth than for others. A second reason is that different facets of adolescent development are on a different time course. While hormonal changes occur at the beginning of adolescence, certain executive functions of the brain are not completely developed until the early 20s. Moreover, different developmental trajectories have been found for different cognitive and emotional processes (Rosso, Young, Femia & Yurgelun-Todd, 2004). A third reason for the difficulty in specifying an exact age range is evidence showing that, particularly in developed countries such as the United States, the onset of puberty is at an earlier average age than seen previously (Parent et al., 2003). At the other end, cultural changes, such as expanding enroll-ment in postgraduate education, have kept young people from assuming adult roles until well into their late 20s. Thus the typical age range of adolescence has been redefined over time, and there are differences in age range between cultures and countries. Regardless of the specific age range of adolescence, the nature of changes in the adolescent brain over time are crucial for understanding why this period of development is particularly important for mental health.