This special section focuses on the incremental validity and utility of clinical assessment data. The lack of replicated incremental validity research limits the ability of psychologists to establish their assessment practices on a solid empirical footing. The articles in this section deal with conceptual, methodological, and content issues in the development and use of clinical measures. The authors addressed several aspects of incremental validity research, including (a) the evaluation of the magnitude of validity increments, (b) the use of incremental validity data for test development and validation, (c) the costs of assessment, (d) the use of multi-informant and multimethod assessments, and (e) the treatment utility of assessment. Obstacles limiting the use of incremental validity research to inform and guide clinical practice are also emphasized.