Promoting marriage, especially among low-income single mothers with children, is increasingly viewed as a promising public policy strategy for improving developmental outcomes for disadvantaged children. Previous research suggests, however, that children's academic achievement either does not improve or declines when single mothers marry. In this article, the authors argue that previous research may understate the benefits of mothers' marriages to children from single-parent families because (1) the short-term and long-term developmental consequences of marriage are not adequately distinguished and (2) child and family contexts in which marriage is likely to confer developmental advantages are not differentiated from those that do not. Using multiple waves of data from the Early Childhood Longitudinal Study-Kindergarten Cohort (ECLS-K), the authors find that single mothers' marriages are associated with modest but statistically significant improvements in their children's academic achievement trajectories. However, only children from more advantaged single-parent families benefit from their mothers' marriage. (Contains 1 figure, 7 tables and 10 notes.)