ADHD is an impairing psychological disorder that predominantly affects children, but also adults to a lesser extent. As a result, a considerable amount of research has been completed in recent years to better understand the nature of the disorder to best treat individuals experiencing symptoms of ADHD. Especially with the publication of the "Diagnostic and Statistical Manual of Mental Disorders" (5th ed.) ("DSM-V") on the horizon, substantial empirical work has been analyzing current ADHD diagnostic criteria and what upcoming changes should be made. Among child and adult populations, adjusting or completely eradicating the age of onset criterion is well supported. With specifically adult populations, amending symptoms to be more developmentally and environmentally appropriate also appears necessary. Even though research additionally questions the state of ADHD subtypes (with both child and adult populations) and other general diagnostic issues (model for diagnosis and level of impairment), continued research is needed to better clarify what other changes should be made in the "DSM-V" and beyond.