In the first part, the difficulties and deficiencies of the DSM-III and DSM-IV diagnostic manuals of the APA, and ICD-9 and ICD-10 of the WHO for this clinical field are discussed. The terms transversal and longitudinal or evolutional, nondirectional and bidirectional, co-morbidity, primary and secondary disorder, clinical co-belonging, concurrence, criteria of validity for clinical constructs, etc., are defined. The exaggerated frequency of co-morbidity is discussed, as well as some of its causes, artifacts and proposals for solution, their usefulness and importance. In the second part are analyzed changes in the different groups and forms of anxiety disorders and the consequences of changes in diagnostic criteria. The third part reviews the rates of prevalence of anxiety disorders in current bibliography. The fourth part reviews the bibliography on the co-morbidity of childhood and adolescent anxiety disorders with regard to the six clinical forms evaluated and mutual morbidity among these forms or types. Finally, the relations between the prevalence of these six types of disorder is examined in terms of age. There is a high frequency of transversal co-morbidity in the overanxiousness disorder and the other types of anxiety disorders, and it is postulated as a longitudinal co-morbidity expression of a possible evolutional relation between separation anxiety disorder and panic disorder with agoraphobia, which in turn is associated with depressive disorders.