The critical neurocognitive components of Attention-Deficit/Hyperactivity Disorder (ADHD) remain controversial, despite extensive research to disclose the essential cognitive components of this disorder. We propose to organize this research into three types of cognitive mechanisms while demonstrating the consistencies and inconsistencies in each: studies of response inhibition, of executive function, and of attentional inhibition. We also present the results of cluster analyses of a group (n = 25) of narrowly selected adult patients with childhood onset of ADHD, reveal three subtypes of ADHD, and argue that the only evidence of a general cognitive deficit is found in tests of selective attention. Finally, we provide the results of exploratory studies of selective attention for perception and for semantic memories. The data suggest that the cognitive disinhibition of ADHD is found in semantic recognition, but not in visuoperceptual recognition.