Disabilities in reading and arithmetic often co-occur, but (dis)abilities in reading and arithmetic have mostly been studied in isolation from each other. This has resulted in two segregated research domains. In this seminar, I will present data in which we explored cross-domain associations and found that early reading and early arithmetic have a unique as well as a shared underlying cognitive basis. I will elaborate on distinct but especially shared cognitive building blocks, as they might help us to understand the co-occurrence of disabilities in reading and mathematics. Further, I want to highlight that researchers focusing on research questions within one specific research domain (especially mathematical cognition) can enrich their work by taking into account (new) scientific insights obtained in the other research domain.