The reliability of the Conners 4 scales were explored in a number of ways through a variety of studies. First, internal consistency was examined with alpha, omega, and mean inter-item correlations, with all values meeting or exceeding typical guidelines for reliability. Standard error of measurement values were provided, demonstrating the small amount of error in estimated true scores, as well as for the facilitation of calculating confidence intervals around the Conners 4 T-scores. Additionally, test information was examined, demonstrating considerable amounts of precision across the scales for each rater type. Further, studies were conducted to assess the test-retest reliability of the Conners 4 scores, with results supporting the stability of scores over a 2- to 4-week period. Lastly, studies were conducted to compare the scores between different raters. Raters of similar types (e.g., two parents) showed similarity in scores, providing evidence for the reliability of the Conners 4, while raters of different types (e.g., parent and youth self-report) showed more modest associations between their scores, emphasizing the best practice of obtaining multiple perspectives when completing an ADHD evaluation. Overall, the results presented within this chapter provide strong evidence for the reliability of the Conners 4 scores.