Dyslexia and attention-deficit/hyperactivity disorder (ADHD) are 2 of the most prevalent complex neurodevelopmental disorders of childhood, each affecting approximately 5% of the population in the United States. These disorders are also each comorbid with speech sound disorder and language impairment. Understanding the nature of the comorbidity among these disorders could lead to advances in developmental theory, a deeper understanding of the genetic and brain mechanisms that cause disability, a more refined diagnostic classification scheme, and new treatments and interventions for children with these disorders. As part of this special issue of "Topics in Language Disorders," this review focuses on the comorbidity between dyslexia and ADHD. It provides a review of the known etiological mechanisms that underlie each disorder. It describes the reconceptualization of these disorders using a multiple deficit model and provides a synopsis of recent studies that illustrate a cohesive approach to investigating the causes of comorbidity. Future directions are discussed in the context of expanding these approaches to the comorbidity among all 4 disorders.