Continued questioning of the validity of a diagnosis of attention-deficit/hyperactivity disorder (ADHD) has created uncertainties about its management in the minds of many clinicians and the public. Inaccurate beliefs about the validity of ADHD hinder the clinical care of many ADHD patients and lead to confusion about the need to seek out or accept treatment. Critics describe ADHD as a diagnosis used to label difficult children who are not ill but whose behavior is at the extreme end of normal. They further contend that, far from having a biological basis, ADHD results from poor parenting and ineffective teaching practices. Such attitudes do much to further stigmatize patients and their families and increase the burden of this debilitating condition. This review attempts to address these challenges by presenting evidence to show that ADHD meets the criteria for a valid psychiatric diagnosis. Not only does it cause specific disabling symptoms that frequently persist into adulthood, but many studies show it has a biological basis and a characteristic response to treatment. Such data support the idea that ADHD is a valid diagnostic category.