Depression is a mood disorder that is characterized by low mood, irritability, less interest in engaging in pleasurable activities, changes in appetite or weight, significant changes in sleep, restlessness or slowing down of cognitive processes or behaviors, excessive tiredness, change in self-esteem, feelings of worthlessness, poor concentration, and suicidal thoughts or recurrent thoughts of death and dying (American Psychiatric Association, 2000). In order to receive a diagnosis of major depressive episode, a subset of these symptoms must be present for 2 or more weeks and must cause significant ­distress or functional impairment in the person’s everyday life. Once an individual has experienced 2 or more major depressive episodes, they are said to have a recurrent major depressive disorder. A diagnosis of dysthymic disorder is defined primarily by a significant level of depressed or irritable mood for at least 1 year that is present in combination with at least two other depressive symptoms. Adolescents with either of these disorders are generally considered to be clinically depressed.

Depression is one of the most common mental health disorders, with point prevalence (i.e., proportion of adolescents who experience depression at any given time) rates for adolescent depression ranging between 2  % and 5  % (see Lewinsohn, Rohde, Klein, & Seeley, 1999 for review) and 12-month prevalence rates ranging from 2  % (McGee et al., 1990) to 13  % (Feehan, McGee, Raja, & Williams, 1994). Findings from the National Comorbidity Survey—Adolescent Supplement (NCS-A; Merikangas et al., 2010) describe significantly higher rates of depression in older adolescents, with lifetime prevalence rates doubling from 8.4  % (at ages 13–14) to 15.4  % (at ages 17–18), with some evidence of up to a 25  % lifetime prevalence by the end of adolescence (Lewinsohn, Rohde, & Seeley, 1998). Significantly more adolescent girls report depression than adolescent boys (15.9  % vs. 7.7  %) (Merikangas et al., 2010). This gender difference first appears between the ages of 13 and 15 years, and then becomes more pronounced between the ages of 15 and 18 years (Hankin et al., 1998). There is some variability in the prevalence rates reported in different studies, as these rates are dependent to a certain degree on the threshold criteria and methodology used by researchers (Boyle et al., 1996; Roberts, Attkisson, & Rosenblatt, 1998).

It is helpful to conceptualize the development of psychopathology within a diathesis–stress model (Abramson, Alloy, & Metalsky, 1988). In this model, an individual has some degree of vulnerability to developing depression (e.g., genetic predisposition, psychological vulnerabilities). This vulnerability, or diathesis, is triggered by stress (either an acute stressor or accumulation of stressors). Each individual’s vulnerability varies, such that a person with a high level of vulnerability may need only a relatively minor stressor to trigger a depressive episode, whereas a person with a low level of vulnerability may need a higher level of stress, or multiple stressors, to trigger a depressive episode. Stressors may include trauma and abuse, academic challenges, interpersonal conflicts, and lack of social support. It is noteworthy that stressors at one point in time may become diatheses at a later point in time. For example, an adolescent may initially experience some form of abuse as a stressor, and over time, this experience may increase that adolescent’s vulnerability by lowering the amount of stress needed to exacerbate depressive symptoms at a later point in time.

Evidence suggests that there may be genetic or biological risk factors for depressive symptoms in adolescents. For example, Van den Bergh and Van Calster (2009) found that adolescents with high levels of depressive symptoms had cortisol profiles that differed from those who had low or moderate levels of depressive symptoms. There is also evidence to suggest that there are significant genetic influences on adolescent mood, which may change over time from childhood to young adulthood (Kendler, Gardner, & Lichtenstein, 2008). Related to this and reflecting possible genetic and environment influences, adolescents who have a family history of depression are at an increased risk for depression (e.g., Silberg, Maes, & Eaves, 2010). Cognitive vulnerability, a consistent pattern of negative thinking or distortions, may also function as a diathesis that increases an individual’s likelihood of experiencing depression (e.g., Abela & Hankin, 2008). Although negative thought patterns are believed to develop over time as a result of interactions with important others (e.g., caregivers), over time they may begin to function as a preexisting vulnerability that is triggered by stress(ors).

Stressors that may trigger a depressive episode, or exacerbate depressed mood, include a history of trauma and abuse (Harkness, Lumley, & Truss, 2008; Suliman et al., 2009). Individuals who have a history of trauma or abuse are at increased risk for developing later depressive symptoms, and evidence suggests that chronicity (or greater number) of abuse or trauma confers even greater risk for poor outcomes (Suliman et al., 2009). Stressors for adolescents may also be school related. This may be either academic stress or difficulties with teachers or classmates. Even when adolescents do not have specific difficulties with academics or peers, they often experience stress during transitions, such as the change from middle school to high school; academic demands increase and changes in peer groups may occur. Finally, depression may be triggered or exacerbated by interpersonal problems, such as conflicts and arguments with parents or peers, being excluded or ostracized from one’s peer group (relational aggression; e.g., Crick, Ostrov, & Werner, 2006), or a lack of social support (Rao, Hammen, & Poland, 2010).

The recurrence rates for depression are relatively high and depression is often cyclical in nature, with episodes frequently triggered by stress(ors). Rates of recurrence during adolescence are approximately 40  % within 2 years (Birmaher et al., 2004) and 70  % within 5 years (Birmaher et al., 1996). Further, there is an increased likelihood of recurrence with each additional episode, as previous depressive episodes confer greater risk for subsequent episodes. There is also evidence that adolescents who experience depression have an increased likelihood of experiencing depressive episodes during adulthood (e.g., Costello, Angold, & Keeler, 1999; Kessler, Avenevoli, & Merikangas, 2001). More recent research suggests that earlier age of onset and recurrence are associated with particularly poor outcomes (Hammen, Brennan, Keenan-Miller, & Herr, 2008).

Additionally, depression in adolescents is often comorbid with other diagnoses. Costello, Mustillo, Erkanli, Keeler, and Angold (2003) found that depressed adolescent girls were 28 times more likely to report an anxiety disorder, ten times more likely to report conduct disorder, seven times more likely to report oppositional defiant disorder, and three times more likely to report substance use. Depressed adolescent boys were 28 times more likely to report anxiety, ten times more likely to report substance use, twice as likely to report attention-deficit/hyperactivity disorder, and 17 times more likely to report oppositional defiant disorder. Compared to other adolescent mental health disorders, depression has the highest comorbidity rate, and this comorbidity appears to affect risk and course of depression, as well as treatment outcomes (see Birmaher et al., 1996 for review).

The two major diagnostic classification systems used to diagnose depression are the International Statistical Classification of Diseases and Related Health Problems (ICD–10; World Health Organization, 1992), which is used internationally, and the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR; American Psychiatric Association, 2000), which is used predominantly in the United States. With either diagnostic system, a thorough assessment needs to be conducted before diagnosing an adolescent with depression.

An evaluation of depressive symptoms in adolescents generally requires a clinical interview with the adolescent (perhaps with a semi-structured diagnostic instrument), a clinical interview with the parent(s), and behavioral observations of the adolescents. It may also include adolescent self-report questionnaires, parent report questionnaires, and teacher report questionnaires (with parent and adolescent consent) (Klein, Dougherty, & Olino, 2005).

The clinical interview with the adolescent is generally considered to be the most important method of assessing depressive symptoms due to the need to understand the adolescents’ level of subjective distress. Parents, however, can provide important information about significant changes in observable behavior, such as sleep, appetite, irritability, and loss of pleasure, as well as about significant life events, transitions, and stressors. Supplemental information about the adolescent’s depressive symptoms and co-occurring conditions, and about the adolescent’s psychosocial strengths, may also be obtained using self-report questionnaires. Finally, behavioral observations of the adolescent during the session, as well as of the adolescent’s interactions with parents or caregivers, can provide valuable information about the adolescent’s depressive affect, level of behavioral activation, and communication patterns. It is important to gather information about the presence of symptoms, as well as their severity (e.g., mild, moderate, severe), duration (i.e., history of length illness and amount of time per day), and frequency (e.g., weekly or daily basis). Given that depression can develop from many different pathways and is often accompanied by co-occurring disorders, collecting broader information such as developmental and social history, family psychiatric history, symptoms of co-occurring disorders, and psychosocial stressors is also an essential part of a thorough assessment of depression.

Finally, typically via the clinical interview, the level of impairment in functioning should be assessed in the domains of school, family, friends, and activities. Although many of the symptoms that are used to assess for depression are internal states, there are several behavioral indicators that may indicate that an adolescent is experiencing depressed mood. These behaviors may include a decline in grades or school performance, changes in peer relationships, behavior problems at home or school, attention difficulties, loss of interest in engaging in regular activities, or withdrawing from family and friends.

It is important to note that depressed mood often manifests as irritability in children and adolescents, as noted in the DSM-IV-TR (American Psychiatric Association, 2000); thus, it is important to carefully discern between depressed mood manifesting as irritability from the presence of both depressed mood and oppositional behaviors. Also, it should be noted that it can be difficult to distinguish between normative adolescent development (e.g., greater independence, less time spent with family) and some depressive symptoms, such as withdrawal and isolative behaviors. It may also be difficult to differentiate some of the symptoms of depressive disorders and alcohol or substance abuse, such as an altered sleep pattern, change in sleeping habits, or declining academic achievement.

When depressive symptoms co-occur with another mental health disorder, such as attention-deficit/hyperactivity disorder, this may result in a more complicated clinical picture. For example, in some instances, the presence of both major depressive disorder and attention-deficit/hyperactivity disorder may resemble bipolar disorder symptoms with depressed mood, irritability, and impulsive emotional reactions. As well, depressive symptoms may be secondary to a medical condition (e.g., depressed mood in reaction to a cancer diagnosis) or to substance use. In these situations, it is important to clarify the timing of the onset of symptoms to determine whether the depression is secondary to another issue.

There is evidence for the efficacy of both cognitive behavioral therapy (CBT; Beck, Rush, Shaw, & Emery, 1979) and interpersonal therapy for adolescents (IPT-A) (Mufson et al., 2004) for treating adolescent depression. Additionally, family therapy may be particularly helpful when conflictual family relationships are closely linked to the adolescent’s mood (Diamond & Josephson, 2005). Finally, there is significant evidence for pharmacological treatment of depressive symptoms, particularly with the use of SSRIs (Cohen, Gerardin, Mazet, Purper-Ouakil, & Flament, 2004). Other treatments, such as electroconvulsive therapy and light therapy, are currently being studied in adolescents. A review of these treatments is as follows:

In this section we use the nomenclature originally developed by O’Carroll et al. (1996) and further revised by Silverman et al. (2007) for defining suicidal ideation and suicide attempt. Accordingly, suicidal ideation is defined as thoughts of killing oneself without regard to intention to act on the thoughts. A suicide attempt is a self-inflicted, potentially injurious behavior with a nonfatal outcome, for which there is evidence (either explicit or implicit) of intent to die. Death by suicide refers to a self-inflicted death with either explicit or implicit evidence of intent to die. Taken together, suicidal ideation, suicide attempt, and/or suicide death will be referred to as suicide–related behaviors.

Adolescent death by suicide is a tragedy of substantial public health significance, both in the United States and throughout the world. Suicide is the third leading cause of death among adolescents and young adults in the United States and the rate of suicide deaths in the United States from 2000 to 2007 (the most recent data available) among adolescents (13–19 years old) was 6.03 per 100,000 (Centers for Disease Control and Prevention, 2010a). In addition to suicide death, suicidal ideation, planning, and attempting are significant public health problems. When asked about suicide-related behaviors in the past year, 13.8  % of high school students report seriously having considered attempting suicide in the past year, 10.9  % made a plan for how they would attempt suicide, 6.3  % attempted suicide, and 1.9  % made a suicide attempt that required medical attention (Centers for Disease Control and Prevention, 2010b).

Although the etiology of suicide-related behaviors is not fully understood, there are multiple risk factors that have been identified for suicide-related behaviors. These risk factors can be organized into the following categories: