We used data from the first wave of the National Longitudinal Study of Adolescent Health to examine family boundary ambiguity in adolescent and mother reports of family structure and found that the greater the family complexity, the more likely adolescent and mother reports of family structure were discrepant. This boundary ambiguity in reporting was most pronounced for cohabiting stepfamilies. Among mothers who reported living with a cohabiting partner, only one-third of their teenage children also reported residing in a cohabiting stepfamily. Conversely, for those adolescents who reported their family structure as a cohabiting stepfamily, just two-thirds of their mothers agreed. Levels of agreement between adolescents and mothers about residing in a two-biological-parent family, single-mother family, or married stepfamily were considerably higher. Estimates of the distribution of adolescents across family structures vary according to whether adolescent, mother, or combined reports are used. Moreover, the relationship between family structure and family processes differed depending on whose reports of family structure were used, and boundary ambiguity was associated with several key family processes. Family boundary ambiguity presents an important measurement challenge for family scholars.