International epidemiologic and clinical studies have shown that comorbid depression and anxiety is of major importance, resulting in more severe symptoms, impairment, subjective distress, and longitudinal course than either anxiety or depression alone. Threshold and subthreshold levels of anxiety ("A," "a") and depression ("D," "d") can be defined on the basis of duration, frequency, impairment, and number of symptoms. These letters can then be used to designate the various combinations of depression and anxiety, e.g., AD, aD. Current evidence demonstrates the importance of evaluating both threshold and subthreshold levels of depression and anxiety.