UNLABELLED Validity and reliability relate to the interpretation of scores from psychometric instruments (eg, symptom scales, questionnaires, education tests, and observer ratings) used in clinical practice, research, education, and administration. Emerging paradigms replace prior distinctions of face, content, and criterion validity with the unitary concept "construct validity," the degree to which a score can be interpreted as representing the intended underlying construct. Evidence to support the validity argument is collected from 5 sources: CONTENT Do instrument items completely represent the construct? RESPONSE PROCESS The relationship between the intended construct and the thought processes of subjects or observers. INTERNAL STRUCTURE Acceptable reliability and factor structure. RELATIONS TO OTHER VARIABLES Correlation with scores from another instrument assessing the same construct. CONSEQUENCES Do scores really make a difference? Evidence should be sought from a variety of sources to support a given interpretation. Reliable scores are necessary, but not sufficient, for valid interpretation. Increased attention to the systematic collection of validity evidence for scores from psychometric instruments will improve assessments in research, patient care, and education.