The present study investigated associations between maternal relationship instability patterns and children's behavioral and emotional functioning in middle childhood in a representative sample of low-income urban families (N = 891). Data from the "Three-City Study" tracked maternal partnerships through the child's life, assessing total marital and cohabiting relationship transitions and delineating transitions by developmental timing, and by directionality (i.e., entrances into versus exits from partnerships). Analyses linking instability to child behaviors at age 8 found that a greater total number of maternal relationship transitions predicted higher anxious, somatic, and conduct problems, with recent transitions (in the prior 2 years) driving these results. Consideration of partnership formations versus dissolutions indicated that recent entrances into new partnerships, and entrances into cohabitations, were most consistently associated with problematic functioning across numerous aspects of children's emotional and behavioral functioning. Policy implications of these findings are discussed.