In this chapter, the psychometric evidence for validity of the Conners 4 was examined. Validity refers to the extent to which the test measures what it is supposed to measure and is studied by exploring the structure of each test and its relations to other measures and to expected outcomes. The Conners 4 is designed to measure symptoms and impairments related to ADHD and related disorders through parent, teacher, and self-reported ratings. To that end, the factor structures of the Content Scales and Impairment & Functional Outcome Scales were explored, finding evidence to support the hypothesized multidimensional models, as well as meaningful interrelations between the factors. Next, scores from the Conners 4 were compared with other measures of ADHD (Conners 3; BASC 3), executive functioning (CEFI), and functional impairment (WFIRS). Correlations were statistically significant and supported the convergence of these measures, providing evidence that similar constructs are measured. Additionally, comparisons were made between clinical groups that were anticipated to display differential profiles of scores on the Conners 4 scales. Youth with ADHD had markedly different results than youth from the general population, and some significant and nuanced differences were reported for youth with Depression, Anxiety, and Disruptive Disorders, supporting the utility of the Conners 4 in distinguishing between clinical diagnoses. Last, the Conners 4 scales were found to successfully classify youth with and without the following diagnoses, demonstrating utility in differential diagnosis decision-making: ADHD, Depression, Generalized Anxiety Disorder, and Oppositional Defiant Disorder. Overall, there is strong evidence for a variety of types of validity to support the intended use of the Conners 4 as a valid measure.