The diagnostic categories of developmental cognitive disorders referred to as attention deficit hyperactivity disorders (ADHD) and learning disorders are relatively vague due to poorly understood etiologies and the construct validity of the diagnostic measures. Consequently, it has been difficult to establish unambiguous diagnostic criteria. Contributing to this diagnostic problem are complications resulting from the co-occurrence of two or more supposedly separate neurocognitive disorders such as comorbidity. Although comorbidity is commonly seen by practitioners and researchers, there is surprisingly little data on the topic. Research and practice can be confounded by the existence of comorbidity. Interestingly, the degree and type of comorbidity in research samples is seldom addressed in the subject descriptions of research reports. This special issue illustrates the nature of comorbidity problems in the case of developmental neurocognitive disorders and especially within and between learning and attention disorders. These problems are often encountered in this area of research and practice, and further discussion is needed concerning the diagnostic criteria and comorbidity.