It is becoming increasingly apparent that although anxiety and depression are separate syndromes and can be identified as such, there is considerable overlap of clinical symptoms and pathophysiological processes. Data indicate that comorbid anxiety and depression is more common than either disorder alone. A large US study found that 58% of individuals with a history of depression also had an anxiety disorder, and a study by the World Health Organization showed that anxiety and depression were the most common coexisting psychological problems in primary care. Generalized anxiety disorder in particular is strongly comorbid with, and commonly precedes, major depression. The implications of comorbid anxiety and depression are significant, with increased social and psychological impairment, and poorer clinical outcomes and prognosis. Anxiety and depression coexist at much higher rates than would be expected by chance alone, suggesting that the two disorders are closely related and may have a common cause. Disturbances of serotonin and norepinephrine neurotransmission are both implicated in anxiety and depression, and new evidence suggests that these systems may provide a mechanistic link between the two disorders, with changes in one system being reflected in the other. Abnormal homeostasis of these two systems may result in anxiety and depression. New theories hypothesize a continuum of illness, with anxiety and depression possibly being different phenotypic expressions of a common neurobiological origin. There is still uncertainty regarding the neurobiological cause, but it is probably linked to dysregulation in the serotonergic and noradrenergic systems.