Because anxiety has normal adaptative forms through development, the identification of pathology may sometimes pose difficulty. Child and adolescent anxiety disorders were defined in modern classifications. Some of them are common to children, adolescents and adults, such as generalized anxiety disorder, panic disorder, simple phobia, social phobia, obsessive compulsive disorder, post traumatic stress disorder. According to DSM IV, separation anxiety disorder is the only disorder which starts during childhood or adolescence. From the epidemiological evidence, anxiety disorders appear frequent. There is evidence of specificity in the course of simple phobia, social phobia and obsessive compulsive disorder, but less specificity in the course of other disorders. Separation anxiety disorder and generalized anxiety disorder represent risk factors for anxiety disorders, depressive disorders or other psychiatric disorders in childhood or adulthood. Data suggest that initial severity or comorbidity may play a role regarding the prognosis of anxiety disorders. Most child and adolescent anxiety disorders are no longer present in adulthood, but most adult anxiety disorders are preceded by child and adolescent anxiety disorders.