I. Introduction
II. Defining Attention-deficit/Hyperactivity Disorder
1. The Progression of DSM-II through IV and ADHD
2. DSM-5 and ADHD
3. Age of Onset of ADHD
4. Heterogeneity of ADHD
5. International Classification Systems for ADHD
III. ADHD Prevalence, Incidence and Epidemiology
1. Prevalence Based on Gender
2. Prevalence Based on Race/Ethnicity
IV. Pathophysiology of ADHD
V. Risk Factors for ADHD
1. Genetic Risk Factors
2. Environmental Risk Factors
3. Interaction of Genetic and Environmental Factors
VI. Presentations of ADHD
1. Distinguishing ADHD Presentations
VII. Summary