This chapter emphasizes that clinical judgments and clinical decision-making involved in activities such as screening, diagnosis, clinical case formulation, and treatment outcome evaluation will be valid to the degree that they are based on science-based assessment strategies and the data obtained with these strategies. The chapter begins with a discussion of diagnostic efficacy (considering prediction ‘hits’ and ‘misses,’ the predictive efficacy or power of a measure) – the degree to which cut-scores accurately identify persons who do or do not manifest some trait or experience some event. Within this context, the sensitivity and specificity of measures are important considerations. The chapter then considers incremental validity in the clinical assessment process – the degree to which the validity of clinical judgments can be improved by adding additional information. The chapter ends with an examination of sensitivity to change – the degree to which measures reflect true changes across time in the measured construct. Imbedded within these sections are considerations of measurement error, the conditional nature of psychometric evidence, and the iterative process of gathering psychometric evidence. The chapter ends with recommendations that emphasize how the nature of clinical decision-making tasks should influence how data from evidence-based assessments are used.