According to the National Institute of Mental Health, (NIMH), Post Traumatic Stress Disorder is one of the 5 recognized types of mental illness commonly known as anxiety disorders. While these five major categories are significant, the complexity of mental disorders demand for segmentation into even more conditions.

The post-traumatic stress syndrome, or PTSD is commonly associated with veterans returning from a war zone.

Major depressive episodes often resolve over time whether or not they are treated. Outpatients on a waiting list show a 10–15% reduction in symptoms within a few months, with approximately 20% no longer meeting the full criteria for a depressive disorder. The median duration of an episode has been estimated to be 23 weeks, with the highest rate of recovery in the first three months.

The three most common treatments for depression are psychotherapy, medication, and electroconvulsive therapy. Psychotherapy is the treatment of choice for people under 18, while electroconvulsive therapy is only used as a last resort. Care is usually given on an outpatient basis, while treatment in an inpatient unit is considered if there is a significant risk to self or others.

Major depressive disorder (also known as clinical depression, major depression, unipolar depression, or unipolar disorder) is a mental disorder characterized by a pervasive low mood, low self-esteem, and loss of interest or pleasure in normally enjoyable activities. The term “major depressive disorder” was selected by the American Psychiatric Association for this symptom cluster under mood disorders in the 1980 version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-III) classification, and has become widely used since.

Behavioral Activation is a third generation behavior therapy for treating depression. It is one of many functional analytic psychotherapies which are based on a Skinnerian psychological model of behavior change, generally referred to as Applied Behavior Analysis. This area is also a part of what is called Clinical Behavior Analysis (CBA) and makes up one of the most effective practices in the professional practice of behavior analysis.