This chapter aims to provide an overview of the typical approaches that have become established in biological and psychological research on anxiety, the questions they address, their findings, and their understanding of the domains of fear, anxieties, and mental anxiety. The biological sciences are primarily concerned with the bodily functions and structures that enable life in a particular environment. In psychology, psychoanalysis, and psychiatry, the focus is on how the psyche operates, how it can be influenced, which mental processes are considered meaningful, and which are experienced as disruptive and distressing. Selected theories that have shaped research in significant ways are briefly introduced.