This chapter covers the cognitive underpinnings of dyslexia and dyscalculia and their co-occurrence, from the perspective of the multifactorial deficit model of developmental disorders (Pennington 2006). The two disorders are first defined with respect to their typical behavioural manifestation. Next, prevalence rates of single as well as co-occurring deficits are described and critically discussed in light of methodological challenges. Subsequently, the cognitive bases of each disorder, as well as their comorbidity, are specified in the light of the multifactorial deficit model. Finally, methodological considerations regarding the link between dyslexia, dyscalculia, and other neurodevelopmental disorders are presented. Future avenues of research are also outlined, with a particular emphasis on treating reading and maths performance as a continuous variable, and on longitudinal studies. The studies presented in this chapter are mainly based on children attending primary school (age range approximately 6–12 years old). There are, of course, differences in the organization, duration, and onset of formal instruction across countries. Aside from nation-specific differences, however, this age period is particularly relevant for research on dyslexia and dyscalculia, as this is the typical time window when literacy and arithmetic skills are acquired.