Measurement error must be taken into account when observations are made during the assessment of human behavior. In classical test theory (CTT), any observed score is equal to the true score of the attribute being measured plus measurement error (Lord & Novick, 1968). Reliability statistics are used to describe the amount of measurement error, which is determined by examining the consistency of measurements obtained across different administrations or parts of the instrument (American Educational Research Association [AERA], American Psychological Association [APA], & National Council on Measurement in Education [NCME], 2014). Reliability can be conceptualized and measured in several ways; however, a common interpretation is that the reliability of an instrument describes how consistent the scores are. Multiple indicators of reliability are provided in this chapter for the Conners Adult ADHD Rating Scale 2nd Edition (CAARS™ 2), including internal consistency, standard error of measurement (SEM), test information, test-retest reliability, and inter-rater reliability. Each indicator is briefly summarized in Key Findings and subsequently explained in greater detail in this chapter. Reliability estimates for the Conners Adult ADHD Rating Scale 2nd Edition Short (CAARS™ 2–Short) and Conners Adult ADHD Rating Scale 2nd Edition ADHD Index (CAARS™ 2–ADHD Index) can be found in chapter 11, CAARS 2–Short, and chapter 12, CAARS 2–ADHD Index, respectively.