The current study is an attempt to inform professionals, such as university trainers and field-based supervisors, as to the utility of professional dispositions assessment as a means of gauging teacher candidate success in their professional roles. According to the "Standards for Educational and Psychological Testing" (2014), test users must select measures that accurately reflect the measurement targets of interest and have validity for intended purposes. Are supervisors' ratings of professional dispositions supported by reliability and validity evidence? Do dispositions make a difference in practice? To address these questions, researchers examined evidence for reliability such as internal consistency, stability, and interrater agreement. To this end, the study explored the structure of the dispositions rating form as to its consistency with its intended structure. The authors examined whether these dispositions are measured consistently across raters and time. Criterion-related evidence for validity was examined by correlating teacher candidate disposition ratings with teaching effectiveness, which was measured by direct observation of their levels of student engagement. Overall, the findings of this study appear to support the skepticism regarding the assessment of dispositions in the teacher education field. The findings challenge the utility of assessing dispositions; that is, unless the term "dispositions" is clearly understood and defined, it cannot be reliably and validly assessed.