Reading disorder frequently co-occurs with other neurodevelopmental disorders and this has important implications for theory as well as practice. Understanding the nature and causes of comorbidities is at the heart of understanding developmental disorders. For example, if Disorder X frequently co-occurs with Disorder Y, this points to the operation of shared risk factors and improves understanding of the causes of each of these disorders. This special issue addresses recent advances in understanding comorbidities between reading disorder/dyslexia and other developmental disorders and identifies issues requiring further research. Comorbidity can be found between disorders within the same diagnostic grouping (homotypic comorbidity: eg, reading disorder and mathematics disorder), as well as between disorders from different diagnostic groupings (heterotypic comorbidity), such as between reading disorder and behavioral disorders (Attention-Deficit-Hyperactivity-Disorder (ADHD) and conduct disorder) or reading disorder and emotional problems (anxiety and depression)(Angold, Costello, & Erkanli, 1999). Rates of comorbidity between reading disorder and other neurodevelopmental disorders range between 11% and 70% for reading disorder and mathematics disorder (Moll, Kunze, Neuhoff, Bruder, & Schulte-Körne, 2014, for an overview), between 20% and 50% for reading disorder and behavioral disorders (ADHD and conduct disorder)(Hendren, Haft, Black, White, & Hoeft, 2018; Hinshaw, 1992; Margari et al., 2013; Sexton, Gelhorn, Bell, & Classi, 2012; Willcutt & Pennington, 2000), and between 9% and 29% for reading disorder and emotional problems (ie anxiety disorder)(Carroll, Maughan, Goodman, & Meltzer, 2005; Margari et al., 2013; Maughan, Rowe, Loeber, & Stouthamer-Loeber, 2003). Although comorbidity rates vary widely between studies, they are clearly consistently higher than expected by chance, indicating that comorbidities between reading disorder and other neurodevelopmental disorders are the rule rather than an exception. However, it is important to note that reading disorder also occurs in isolation, as do other neurodevelopmental disorders. This raises questions about how the risk factors underlying reading and other neurodevelopmental disorders interact and how to explain the different behavioral patterns. A comprehensive model of reading disorder (or any other developmental disorder) needs to be able to explain not only the disorder itself but also its co-occurrence with other disorders (see Landerl, Göbel, & Moll, 2013 for a similar discussion on mathematics disorder). Nevertheless, most research over the last few decades has focused on single disorders, neglecting co-occurring features. In fact, some studies have deliberately excluded individuals with additional problems (notably, problems in attention) in order to identify the deficits that are specific to a given disorder. A major step toward a better understanding of developmental disorders and their comorbidities was the shift from single deficit models to multiple deficit models, first discussed in the groundbreaking work by Bruce Pennington (2006). While earlier research based on single-deficit models had helped to identify the deficits that are specific to a given disorder (such as reading disorder), it became clear that single-deficit approaches suffer from several shortcomings and a new theoretical framework was required to account for the complex etiology of neurodevelopmental disorders