Objective. The purposes of this study were to identify mother, child, and dyadic determinants of effective mother–child collaboration and to determine the impact of this collaboration on children's cognitive development. Design. Ninety-two mother–child dyads from the Massachusetts site of the National Institute of Child Health and Human Development Study of Early Child Care and Youth Development participated in a site-specific, home-based instructional task through which they were assessed for scaffolding effectiveness. Cognitive characteristics of both mothers and children, as well as dyadic characteristics from infancy, were examined as predictors of effective dyadic scaffolding when the children were in 1st grade. In addition, concurrent cognitive capabilities of the children were regressed on scaffolding while controlling for earlier cognitive test scores. Results. Mothers' verbal intelligence and children's mental development, as well as shared sensitivity, predicted the effectiveness of scaffolding collaborations, which in turn uniquely predicted cognitive capabilities of the children. Conclusions. Effective mother–child scaffolding is a function of individual mother and child characteristics, as well as the nature of the mother–child relationship; scaffolding predicts children's cognitive outcomes.