Approximately 3-6% of school-age children are estimated to have mathematics difficulties. There are many more children in regular school classrooms who struggle with mathematics, but whose performance is not considered sufficiently poor to be classified as meriting a specific disability in mathematics. Specific mathematic learning disability (MLD) is defined in psychiatric and educational venues. One issue that complicates the study of the cognitive underpinnings of mathematics proficiency is how mathematic abilities are assessed. Regardless of whether preschool, school-age, or adolescent students are studied, mathematic proficiencies can be measured by traditional, individually administered standardized achievement tests. There may be important differences in the identified cognitive substrates dependent on the measurement method. The chapter provides diverse evidences relating WM and various mathematic skills, which were measured by different methods. Cognitive limitations in childhood do lead to difficulties in learning basic arithmetic and mathematic skills, and these cognitive limitations need to be pinpointed to help children in their learning. Mathematic skills are supported by verbal and visual-spatial STM resources, the reliance on which may vary depending on age and experience. To identify children at risk of developing mathematic difficulties, combination of subject specific tasks and general WM tasks can be used. A deeper understanding of the cognitive limitations will help one to develop teaching strategies to overcome or circumvent these difficulties.