This chapter presents information on the conditional nature of psychometric evidence. There is a large body of evidence indicating that the psychometric properties and clinical utility of the measures derived from an assessment instrument are conditional and unstable: this includes evidence for reliability, validity, utility, and factor structures. Psychometric evidence of the measures that guide clinical judgment and clinical decision-making can vary as a function of the context and characteristics of an assessment occasion and the persons from whom the measures are derived. Further, the elements of some clinical assessment instruments can diminish over time in their relevance to and representativeness of the target construct as our understanding of the targeted phenomenon increases. This chapter considers types of incremental validity, methods for evaluating incremental validity, and the utility of new assessment instruments and measures. Subsequent sections discuss the multifaceted nature of many clinical phenomena and assessment instruments and reviews how psychometric evidence can vary with the clinical population, state of the client when assessed, psychometric dimension of the measure, severity of the disorder, assessment setting and context, goals of assessment, the measures derived from an instrument, and the temporal focus of assessment. The chapter ends with recommendations for conducting clinical assessments and interpreting their results in the context of the conditional and dynamic nature of psychometric evidence.