Research Findings: Children from families of lower socioeconomic status (SES) enter kindergarten with less developed mathematical knowledge compared to children from middle SES families. This discrepancy is present at age 3 years and likely stems from differences in the home learning environment. This study reports SES-related differences both in the quantity and quality of mathematical support children receive in the home and in parent beliefs about early mathematical development and then compares both with children's performance on a comprehensive mathematics assessment. Participants included 90 children in their 1st year of preschool (2 years before kindergarten entry) and 88 children in their prekindergarten year (the year just prior to kindergarten entry). Both cohorts were balanced for SES and gender. The results suggested minimal SES-related variation in mathematical support received in either cohort but clear SES differences in parents' beliefs about early mathematical development. Middle SES parents of children in both cohorts held higher expectations in terms of skills they expected children to possess by age 5, as well as a more accurate understanding of which skills are within the developmental range of most children by age 5. These differences accounted for unique variance in children's scores on the mathematics assessment. Practice or Policy: Implications are discussed.