This project focuses on children with arithmetical difficulties. The purpose is to examine whether working memory deficits are responsible for these children's arithmetical difficulties. Working memory is a mental structure that consists of three different components: a central attentional-controlling component, and two components that are specialised in processing and storing verbal information and visual-spatial information, respectively. The study will include four groups of children: 1) Children with specific arithmetical difficulties, but adequate reading skills (3rd and 4th graders), 2) Children with both arithmetical and reading difficulties, and two control groups; one age-matched control group (3rd and 4th graders) and one group of younger ability-matched children (2nd and 3rd graders). A battery of cognitive tasks that measure different aspects of the working memory system will be used and administrated to the children. In order to determine what aspects of the working memory system that are impaired performance of the children with arithmetical difficulties will be compared to the age-matched children's performance. By comparing working memory performance with the younger ability-matched children (2nd and 3rd graders), it will be possible to determine whether working memory deficits are responsible for these children's arithmetical difficulties.