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. It remains true that the primary goal of scale development is 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 (a) clear conceptualization of target constructs, (b) an overinclusive initial item pool, (c) paying careful attention to item wording, (d) testing the item pool against closely related constructs, (e) choosing validation samples thoughtfully, and (f) emphasizing unidimensionality over internal consistency. We have added (g) consideration of the hierarchical structures of personality and psychopathology in scale development, discussion of (h) codeveloping scales in the context of these structures, (i) "orphan," and "interstitial" constructs, which do not fit neatly within these structures, (j) problems with "conglomerate" constructs, and (k) developing alternative versions of measures, including short forms, translations, informant versions, and age-based adaptations. Finally, we have expanded our discussions of (l) item-response theory and of external validity, emphasizing (m) convergent and discriminant validity, (n) incremental validity, and (o) cross-method analyses, such as questionnaires and interviews. We conclude by reaffirming 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. (PsycINFO Database Record (c) 2019 APA, all rights reserved).