Research is now suggesting that intolerance of uncertainty may be very important in understanding worry and may play a key role in the etiology and maintenance of worry. The present study attempted to further our understanding of intolerance of uncertainty by examining the psychometric properties of the English version of the Intolerance of Uncertainty Scale (IUS), which has already been validated in French. Factor analysis indicated that the IUS has a four-factor structure that represents the idea that uncertainty is stressful and upsetting, uncertainty leads to the inability to act, uncertain events are negative and should be avoided, and being uncertain is unfair. The IUS has excellent internal consistency, good test-retest reliability over a five-week period, and convergent and divergent validity when assessed with symptom measures of worry, depression, and anxiety. Overall, this study suggests that the IUS is a sound measure of intolerance of uncertainty and supports the idea that intolerance of uncertainty is an important construct involved in worry.