Attention-Deficit/Hyperactivity Disorder (ADHD) in adults can be significantly impairing across various life domains. As manifestations of ADHD in adulthood become increasingly recognized, however, research has started to address diagnostic challenges inherent in the clinical evaluation of adult ADHD. One such challenge is the reliance on self-reported ratings when measuring functional impairment associated with ADHD symptomatology. Considering the high clinical need for accurate assessments of functional impairment in the diagnosis of ADHD, it is critical that both clinicians and researchers understand the falsifiability of subjective instruments commonly used to measure functional impairment. We used secondary analysis of data from two cross-sectional studies to investigate the relations among perceived functional impairment, symptom validity, and performance validity in adults presenting with ADHD. Study 1 utilized a simulation study design with a general functional impairment measure, while Study 2 was a clinical known-groups study with an ADHD-specific functional impairment measure. As expected, noncredible responders reported significantly greater symptoms of inattention, hyperactivity-impulsivity, anxiety, and depression compared to credible responders across both studies, though there were variable findings for self-reported somatic symptoms. Additionally, noncredible responders reported greater overall functional impairment relative to credible responders, and a greater proportion of noncredible responders endorsed clinically impaired levels of functional impairment. While noncredible presentations significantly moderated the association between self-reported functional impairment and hyperactive-impulsive symptoms in Study 1, there was less consistent support for a moderating effect of noncredible presentations on the relationship between symptoms and impairment in Study 2. These results suggest that, in adult ADHD evaluations, noncredible presentations can significantly impact measures of perceived functional impairment. The vulnerability of self-reported impairment ratings to noncredible presentations highlights the need for assessors to include measures of symptom validity and performance validity in adult ADHD evaluations. [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.]