The purpose of this chapter is to provide a comprehensive review of what is known about the etiology and pathology of anxiety disorders across the lifespan, highlighting in particular developmental differences in the expression of individual anxiety disorders. Anxiety is implicated heavily across the full range of psychopathology. Anxiety is a future‐oriented emotion characterized by marked negative affect, bodily symptoms of tension, and chronic apprehension. Fear, on the other hand, is an immediate alarm reaction to present danger characterized by strong escapist action tendencies. Anxiety, fear, and panic are the building blocks of anxiety disorders, arranging themselves in different ways as they focus on varying internal and external stimuli that have become imbued with threat or danger to form the commonly recognized variants of anxiety disorder. Included within this chapter will be separation anxiety disorder, obsessive‐compulsive disorder, specific phobias, social phobia, panic disorder (with and without agoraphobia), and generalized anxiety disorder. Covered for each disorder will be clinical presentation, prevalence, demographics, comorbidity, cultural influences, developmental changes, and etiology.