This chapter explores the developmental dyscalculias. Some of the interest in developmental dyscalculia has arisen because of the arithmetical problems sometimes seen in developmental dyslexia and the literature is skewed towards studies of children in whom the conditions co-occur. The nature of the mechanisms that children must acquire for arithmetic computation and their representation in the brain has been discussed in relation to classical neuropsychological studies of acquired dyscalculia. The potential dissociations of arithmetical skill from both intelligence and reading level were emphasised by the existence of a proportion of children with specific reading difficulties for whom arithmetical skills were developing well. The basis of arithmetical competence must begin before the child starts at school. Recent cognitive neuropsychological analyses of the acquired dyscalculias have distinguished between disorders of number processing, number facts, and procedural knowledge. The number processing system comprises the mechanisms for perceiving, comprehending, and producing numbers.