The nature of the world in which mammals have evolved has resulted in an attentional system that is highly selective. It is obvious that natural environments consist of multiple objects, most of which also contain many different attributes. For example, a walk down a typical high street bombards us with a variety of sounds, colours, and odours. In addition to these external objects, there may also be a range of competing internal thoughts that occupy our minds. Because of this huge variety of stimuli, it seems clear that the ability to efficiently select and focus on currently relevant information is critical for coherent behaviour. In addition, it would also seem to be adaptive to rapidly notice stimuli that may pose a danger. For instance, it is important to notice the predator or potential mugger lurking in the nearby bushes. Therefore, in addition to the necessity for selection of relevant objects and thoughts, an efficient attentional system must also be open and flexible to stimuli that may represent a threat. Thus, there may well be a subset of stimuli that has privileged access to the attentional system. In addition to certain classes of stimuli that may be especially salient in terms of attracting attentional resources (e.g., snakes, spiders, angry facial expressions, etc.), there may be groups of individuals who are particularly sensitive to threat-related stimuli. Indeed, anxiety has often been characterized as the quintessential example of a hypervigilant attentional system.