Children in the bottom quartile of mathematics achievement are at high risk for underemployment in adulthood. These children include the roughly 7% of students with a mathematical learning disability (MLD) and another 10% of students with persistent low achievement (LA) that is not attributable to intelligence. The poor mathematics achievement of children who compose groups of MLD and LA students appears to be related to one or several deficits; specifically, (1) a delay in the development and poor fidelity of the system for representing approximate magnitudes; (2) difficulty mapping Arabic numerals, number words, and rational numbers onto associated quantities; (3) poor conceptual understanding of some arithmetic concepts; (4) developmental delay in the learning of mathematical procedures; and (5) difficulty committing basic arithmetic facts to or retrieving them from long-term memory. Children with MLD also have concurrent working memory deficits that exacerbate their mathematics-specific deficits and delays.