The concept of ambiguity tolerance (TA), variously called Uncertainty Avoidance, Ambiguity Avoidance, or Intolerance, can be traced back nearly 70 years. It has been investigated by many different types of researchers from clinical and differential, to neuro- and work psychologists. Each sub-discipline has tended to focus on how their variable relates to beliefs and behaviors in their area of expertise, from religious beliefs to reactions to novel products and situations. The basic concept is that people may be understood on a dimension that refers to their discomfort with, and hence attempts to avoid, ambiguity or uncertainty in many aspects of their lives. There have been many attempts to devise robust and valid measures of this dimension, most of which are highly inter-correlated and require self-reporting. There remains a debate as to whether it is useful having just one or more dimensions/facets of the concept. Using these tests, there have been many correlational studies that have sought to validate the measure by looking at how those high and low on this dimension react to different situations. There have also been some, but many fewer, experimental studies, which have tested very specific hypotheses about how TA is related to information processing and reactions to specific stimuli. There is now a welcomed interest by neuroscientists to explore the concept from their perspective and using their methodologies. These studies have been piecemeal, though most have supported the tested hypotheses. There has been less theoretical development, however, of the concept attempting to explain how these beliefs arise, what sustains them, and how, why, and when they may change. However, the concept has continued to interest researchers from many backgrounds, which attests to its applicability, fecundity, and novelty.