This chapter gives an overview of the clinical presentation of anxiety disorders and reviews basic epidemiological knowledge on them. The presented knowledge is largely related to the classification of anxiety disorders as presented by the Diagnostic and Statistical Manual of Mental Disorders since itsthird revision (DSM-III). Without going into detail into the history of the classification of anxiety disorders and into the history and development of the several editions of the Diagnostic Manual of Mental Disorders (DSM) of the American Psychiatric Association (APA) it should just briefly be mentioned that the DSM of the APA has undergone until today four revisions. Within these revisions, the third edition (DSM-III) changed most radically from the forerunning ones. The major change in DSM-III was that the category "anxiety neurosis" was deleted because this term was too general and could not be defined reliably. On the basis of evidence that imipramine can block panic attacks, panic disorder was created as a new diagnosis for the first time in DSM-III. Anxiety states without spontaneous panic attacks were separated from panic disorder and defined as a residual category, generalized anxiety disorder. The revised version of DSM-III, DSM-III-R, was published in 1987, and the fourth and most recent edition, DSM-IV, was published in 1994. More recently, a text revision of DSM-IV has been published that does not entail changes to the diagnostic criteria of disorders, but provides updated empirical reviews for each diagnostic category regarding associated features, cultural, age, and gender features, prevalence, course, familial patterns, and differential diagnosis (DSM-IV-R). Without going into further details of the development and changes across the different editions and revisions of DSM--these have been reviewed comprehensively in other reviews--this chapter gives an overview about the clinical presentations of anxiety disorders by referring mainly to the forth edition of the DSM (DSM-IV 1994). In the second part, the chapter reviews and summarizes selected aspects (prevalence, correlates, risk factors and comorbidity) of epidemiological knowledge on anxiety disorders.