Main and interactive effects of child care quality, stability, and multiplicity on infants' attachment security, language comprehension, language production, and cognitive development at 15 months were examined using data from the National Institute of Child Health and Human Development (NICHD) Study of Early Child Care. Thirty-nine percent of the infants in this sample experienced arrangement change, and 46% experienced multiple concurrent arrangements during the first 15 months. As in previous studies, concurrent quality, average quality, and quality slope significantly predicted cognitive and language development. There was some evidence that certain forms of unstable child care--including nonfamilial change, familial to nonfamilial change, and within-home to out-of-home change--were associated with poorer language development. Multiple child care arrangements involving family members positively predicted language comprehension; multiple care involving a mix of family and nonrelative caregivers negatively predicted language comprehension. Interactions among variables exhibited "effects in context." That is, under conditions of low or moderate quality in the primary care arrangement, the use of fewer multiple arrangements was associated with higher language scores; under conditions of high primary care quality, the use of more multiple arrangements was associated with higher language scores.