This chapter describes the prevalence, onset, course, persistence, comorbidity, and outcome, as well as correlates and risk factors of anxiety disorders, namely separation anxiety disorder, specific phobia, social anxiety disorder, agoraphobia, panic disorder, and generalized anxiety disorder. The focus is laid upon the early years of life (childhood, adolescence, and young adulthood), given that most anxiety disorders have their onset at this time, typically persisting over the life course, and thus representing powerful risk factors for the onset of subsequent mental disorders such as depression and substance use disorders. Despite progress, continued research efforts are needed towards identifying which vulnerability and risk factors play a causal role for the onset and persistence of pathological anxiety. An improved understanding of the complex underlying biological and psychological mechanisms and interactions is crucial to facilitate more effective targeted prevention research and treatment.