The authors explicated the validity of the Inventory of Depression and Anxiety Symptoms (IDAS; D. Watson et al., 2007) in 2 samples (306 college students and 605 psychiatric patients). The IDAS scales showed strong convergent validity in relation to parallel interview-based scores on the Clinician Rating version of the IDAS; the mean convergent correlations were 0.51 and 0.62 in the student and patient samples, respectively. With the exception of the Well-Being Scale, the scales also consistently demonstrated significant discriminant validity. Furthermore, the scales displayed substantial criterion validity in relation to "Diagnostic and Statistical Manual of Mental Disorders" (4th ed.; "DSM-IV"; American Psychiatric Association, 1994) mood and anxiety disorder diagnoses in the patient sample. The authors identified particularly clear and strong associations between (a) major depression and the IDAS General Depression, Dysphoria and Well-Being scales, (b) panic disorder and IDAS Panic, (c) posttraumatic stress disorder and IDAS Traumatic Intrusions, and (d) social phobia and IDAS Social Anxiety. Finally, in logistic regression analyses, the IDAS scales showed significant incremental validity in predicting several DSM-IV diagnoses when compared against the Beck Depression Inventory-II (A. T. Beck, R. A. Steer, & G. K. Brown, 1996) and the Beck Anxiety Inventory (A. T. Beck & R. A. Steer, 1990). (Contains 9 tables and 3 footnotes.)