Anxiety is a common, costly and growing problem in our society. Despite considerable heritability and being a primary cause of lifelong disability, little is known about the molecular basis of anxiety. However, experimental psychology has developed exceptionally robust paradigms that model in both animals and humans how anxiety (fear) is developed (or learned) and how it is treated (or extinguished). No study has attempted to apply to combine modern genetic approaches with the use of these experimental paradigms at scale, with sufficient power to allow transformative science. We propose to conduct the largest systematic genetic study of an experimental psychology paradigm - fear learning and extinction - in order to significantly advance our understanding of mechanisms underlying the development and treatment of anxiety.We will also develop genetic tools for use in predicting treatment outcome, paving the way for personalised treatment decisions for anxiety disorders.