In this update of Clark and Watson (1995), we provide a synopsis of major points of our earlier article and discuss issues in scale construction that have become more salient as clinical and personality assessment has progressed over the past quarter-century. The primary goal of scale development is still to create valid measures of underlying constructs and that Loevinger’s theoretical scheme provides a powerful model for scale development. We still discuss practical issues to help developers maximize their measures’ construct validity, reiterating the importance of (1) clear conceptualization of target constructs; (2) an over inclusive initial item pool; (3) paying careful attention to item wording; (3) testing the item pool against closely related constructs; (4) choosing validation samples thoughtfully; and (5) emphasizing unidimensionality over internal consistency. We have added (6) consideration of the hierarchical structures of personality and psychopathology in scale development; discussion of (7) co-developing scales in the context of these structures; (8) “orphan,” and “interstitial” constructs, which do not fit neatly within these structures; (9) problems with “conglomerate” constructs; and (10) developing alternative versions of measures, including short forms, translations, informant versions, and agebased adaptations. Finally, we have expanded our discussions of (10) item-response theory and of external validity, emphasizing (11) convergent and discriminant validity, (12) incremental validity, and (13) cross-method analyses, such as questionnaires and interviews. We conclude by re-affirming that all mature sciences are built on the bedrock of sound measurement and that psychology must redouble its efforts to develop reliable and valid measures.