ADHD affects over six million children in the U.S. and despite its prevalence, many clinicians do not accurately diagnose ADHD and do not screen for and identify the numerous conditions that can coexist and even worsen true ADHD or cause ADHD-like presentations when it does not exist. To help clinicians, this book offers three components. Part 1 presents the ADHDology Evaluation Model, which provides the ten steps to comprehensively evaluate ADHD. Part 2 presents numerous medical, sleep, psychological, trauma, neurodevelopmental, sensory processing, and fetal substance exposure conditions. These chapters describe the conditions in detail, how they coexist with or appear similar to ADHD, how to distinguish them from true ADHD, and how mental health clinicians and specialists can further evaluate and treat these disorders. Part 3 is composed of the Comprehensive Diagnostic ADHD Screening System (CDASS), a unique approach to improve the accuracy of evaluating ADHD by utilizing checklists to help identify: the risk factors associated with ADHD, the many possible conditions presented in Part 2 that may exist so these can be further considered and evaluated by specialists, and little-known and not typically considered conditions that can cause ADHD-like presentations. While written mainly for clinicians; parents, educators, and interested others will find the text helpful to better understand these complex topics, as well as assist clinicians with the ADHD diagnostic process.