The risk of improbable, uncertain, but grave potential dangers poses unique adaptive challenges. We argue that to manage such risks, a special motivational system evolved, which we term the security motivation system. Review of work across a range of species indicates that this system is designed to detect subtle indicators of potential threat, to probe the environment for further information about these possible dangers, and to motivate engagement in precautionary behaviors, which also serves to terminate security motivation. We advance a neurobiological-circuit model of the security motivation system, which consists of a cascade of cortico-striato-pallido-thalamo-cortical loops with brainstem-mediated negative feedback. We also detail the broader physiological network involved, including regulation of the parasympathetic nervous system, with emphasis on vagal regulation of cardiac output, and activation of the hypothalamic-pituitary-adrenocortical axis. Finally, we propose that some kinds of psychopathology stem from dysfunction of the security motivation system. In particular, obsessive compulsive disorder may result from the failure of a mechanism by which engagement in precautionary behavior normally terminates activation of the system.