Continuous performance tests (CPTs) are frequently used to measure attention and impulsivity in children and adults during psychological and neuropsychological evaluations, often to inform differential diagnosis of Attention-Deficit Hyperactivity Disorder (ADHD). Despite the widespread clinical use of CPTs, the majority of research on their utility in a diagnostic context has not been conducted using ecologically-valid samples. The present study utilizes an ADHD group that includes individuals who have both ADHD and other comorbid psychiatric conditions, and compares them to both a healthy control group, and a clinical control group comprised of individuals with other psychiatric disorders. Scores from the Quotient CPT were used to predict diagnostic group. Quotient scores were also correlated with cognitive and psychological questionnaire data. Participants (ages 5-21) data were previously collected throughout New York City through the Healthy Brain Network study. Psychiatric diagnosis was determined via semi-structured interview by a licensed clinician and subsequent consensus case conference (independent of other data collected in the study). The Quotient System Index was associated with diagnostic group (ADHD, healthy control, or mixed clinical group). Furthermore, all three Quotient subscale scores significantly varied between the ADHD group and the mixed clinical group and the ADHD group and the healthy control group. Although CPT performance was associated with diagnostic group, it did not add significant incremental validity in a linear regression with other gold standard measures of ADHD included. In other analyses, CPT performance generally correlated with a number of measures, including questionnaire subscales and cognitive measures. Findings support that the Quotient CPT adequately distinguishes between its intended target group--ADHD--compared to mixed clinical and healthy control groups. However, psychological and neuropsychological evaluation necessitates a balance between cost (i.e., time and money) and performance (i.e., incremental validity) and results from this study suggest that the CPT may not meet this threshold. Nevertheless, it may have use in other contexts, including research of cognition in ADHD. Future research on its clinical utility could examine the performance of other CPTs in ecologically valid samples, and the utility of CPT scores in distinguishing between and diagnosing the subtypes of ADHD. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]