The two distinguishing hallmarks of anxiety disorders are an emotional state of fear and worry that result in diminished functioning. Fearfulness may be based on an actual experience or fostered by cognitive distortions that result in the misperception of a threat. Consequently, a physiological response occurs that ranges from disconcerting to debilitating and may result in aggressive or avoidant behaviors to escape the distress. The worry associated with anxiety creates a persistent state of angst or apprehension that is sustained over time. Without treatment, anxiety can negatively impact personal well-being, academic achievement, employment, and lifelong accomplishments. This entry discusses types of anxiety disorders as well as measurement options.