Fifty years ago, L. J. Cronbach and P. E. Meehl (1955) advocated for the concept of construct validity, noting that psychologists study hypothetical, inferred entities and that validating measures of such entities involves basic theory testing. Three important developments in clinical assessment following that seminal article are noteworthy. First, clinical research has benefited from greater theoretical integration and subsequent differentiation among related constructs. Second, implementation of ongoing, critical evaluation of all aspects of the construct validity process, including theory development, hypothesis specification, research design, and empirical evaluation, has improved clinical assessment. Third, improvement in evaluating fit between hypotheses and observations has been sought. Improved means of evaluating multitrait, multimethod designs, and ways to increase their clinical representativeness, are one encouraging development. Ongoing efforts to improve the construct validity process reflect the legacy of L. J. Cronbach and P. E. Meehl.