Growing agreement exists among psychologists that all traits are not equally relevant to allpeople. The assumption that traits differ in their relevance across individuals has implications for psychometric theory and the assessment of reliability and validity. Specifically, the less relevant a trait is to an individual, the more error that person is contributing to the measurement process. This article discusses the construct of trait relevance, explicates why differences in trait relevance need to be incorporated into models of trait assessment, and suggests new possibilities for the measurement of trait relevance.