This article discusses the use of dispositions as a basis for evaluating candidates for teaching credentials. It begins with the assumption that any evaluation process must rely on clearly defined constructs that cannot be interpreted in open-ended ways to suit the subjective biases of the evaluator. It then argues that present uses of the disposition construct in teacher education risk this kind of biased application. It makes the point that fairness in assessment procedures requires rigorous and unambiguous definitions of assessment standards by criteria drawn from science and other systematic areas of scholarship. A definition of disposition drawn from the behavioral sciences is offered, and principles for using dispositions as a standard in teacher assessment are offered. (Contains 2 notes.)