The construct of perfectionism is related to many important outcome variables. However, the term "perfectionism" has been defined in many different ways, and items comprising the different existing scales appear to be very different in content. The overarching aim of the present set of studies was to help clarify the specific unidimensional personality constructs that contribute to perfectionistic behavior. First, trained raters reliably sorted items from existing measures of perfectionism into nine dimensions. An exploratory factor analysis, followed by a confirmatory factor analysis on an independent sample, resulted in a 9-scale, 61-item measure, called the Measure of Constructs Underlying Perfectionism. The nine scales were internally consistent and stable across time, and they were differentially associated with relevant measures of personality in theoretically meaningful ways. (Contains 7 tables.)