This paper serves to provide a background for the topic of comorbidity than extends through this issue. Comorbidity is common within developmental disorders. It is shown that there are many possible reasons for comorbidity. 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.