Anxiety disorders are the most common class of psychiatric illnesses. These conditions typically develop in adolescence and young adulthood and tend to run a chronic course. Anxiety disorders are underdiagnosed and undertreated by physicians, and knowledge gaps and complex symptom manifestations partly account for this. Further, affected individuals often do not seek help until later and more severe stages in the disease process. To synthesise the fragmented research on the prevalence of anxiety disorders, we did a systematic review of reviews. Evidence from the highest quality systematic reviews was included to inform future research and health-care programmes.