It is important to understand the neurobiological, cognitive, and behavioral factors that underlie ADHD and the ADHD subtypes. It has often been hypothesized that the ADHD-Combined Type (ADHD-CT) and ADHD-Predominantly Inattentive Type (ADHD-PI) subtypes may be the result of different underlying factors and may be associated with different types of attentional deficits. This work is an attempt to examine possible ADHD subtype differences by testing hypothoses based on cognitive theories of attention, research findings on the neurobiology of attention and ADHD, as well as previous research findings on and theories of ADHD subtyping. The attentional processes discussed in this work include response activation, sustained attention, encoding/working memory, the focus/execute aspect of attention, and attentional stability.