The psychometric evidence presented in this chapter provides strong support for the validity of the CAARS 2. 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 how it relates to other measures and to expected outcomes (APA, AERA, & NCME, 2014). The CAARS 2 is designed to measure symptoms and impairments related to ADHD and related disorders through self-reported and observer ratings. With respect to factorial validity as reflected in a measure’s internal structure, multiple analyses indicated strong fit statistics for the chosen, conceptually sound, CAARS 2 5-factor model, with meaningful interrelations between the five distinct factors. With respect to construct validity, correlations between the CAARS 2 and other established tests, including measures of ADHD (CAARS), executive functioning (CEFI Adult), and functional impairment (WFIRS-S), were statistically significant and provided evidence that similar constructs are measured by these related instruments. Evidence of criterion validity emerged from analyses demonstrating statistically significant and meaningful differences in average scale scores between both General Population and ADHD groups and between ADHD and other clinical groups. Individuals with ADHD had markedly higher scores and more item-level endorsement than individuals from the general population, as well as meaningfully different profiles of results as compared to individuals with Depression and/or Anxiety, supporting the utility of the CAARS 2 in distinguishing between clinical diagnoses. Additionally, the CAARS 2 scales and Impairment & Functional Outcome Items were found to successfully classify individuals with and without ADHD, demonstrating utility in differential diagnostic decision-making. Overall, the strong evidence for multiple types of validity supports the intended use of the CAARS 2 for assessing ADHD in adults.