Mastering a basic understanding of numbers is critically important for children’s future success in school, the workplace and everyday life. Unfortunately, individual differences in children’s understanding of basic number concepts arise early in development and gaps in number knowledge are particularly apparent amongst children of varying socioeconomic statuses. Research in home and school environments suggests that much of the variability in children’s number knowledge can be attributed to differences in quantity and quality of the number-related input that children receive from parents, teachers and caregivers. The present chapter explores sources of variability in children’s understanding of numbers, factors that make learning about numbers challenging, and characteristics of effective number input. Finally, we discuss how previous and ongoing research can be useful in informing policies aimed at promoting early number knowledge and reducing achievement gaps.