There is a national need for effective interventions to improve school readiness and subsequent achievement in mathematics for students from low-income families. The purpose of this study was to implement and evaluate a 2-year preschool math intervention that began at preschool entry when children were 3 years of age and continued through the end of the pre-kindergarten (pre-K) year. Three principal objectives will be addressed in this presentation: (1) to evaluate the efficacy of a math curriculum for 3-year-olds implemented in the pre-pre-kindergarten (pre-pre-K) year of preschool; (2) to compare the impact of a 2-year math intervention (implemented during pre-pre-K and pre-K years) with a 1-year math intervention (PK) or a business-as-usual control condition on children's mathematical knowledge at the end of preschool; and (3) to examine the longitudinal effects of the 2-year and 1-year math interventions on young children's mathematical development in kindergarten. The study was conducted in 63 classrooms in three Head Start programs in Northern California. Children remained in the same classrooms with the same teachers throughout their two years of preschool. The study concluded that the Pre-Pre-K Mathematics intervention had a large impact on 3-year-olds' mathematical knowledge. Likewise, the Pre-K Mathematics intervention had a significant impact on 4-year-olds' mathematical knowledge. The cumulative impact of receiving 1 vs. 2 years of math intervention appeared to be surprisingly similar at the end of pre-K. Gains children made during the 1-year intervention began to fade in K, but gains children made during the 2-year intervention were sustained in K. Possible explanations are better retention after 2 years of intervention or greater consolidation of previously learned informal math knowledge. Figures are appended.