This article provides a background for the topic of comorbidity of attention-deficit/hyperactivity disorder and spoken and written language and speech disorders that extends through this issue of "Topics in Language Disorders." Comorbidity is common within developmental disorders and may be explained by many possible reasons. Some of these can be viewed as artifacts as simple as chance occurrence or because of the way that the research participants were sampled. If these artifacts are eliminated, then comorbidity can be informative with respect to possible causes of the disorders that are comorbid. Several possible etiologic models are presented along with a general framework for considering levels of causality in developmental disorders. (Contains 2 figures and 2 tables.)