Although the family continues to be a critical unit in demographic and social analysis, perceptions of what constitutes the ''family'' vary across groups and societies. The standard definition of the family used in U.S. censuses and surveys (persons related by blood, marriage, or adoption, and living in the same residence) may limit description and analysis of family structure. Yet, it is what determines official data on the family. Because information on alternative family definitions is not available for the U.S., we use data from the Netherlands Kinship Panel Study to assess the effect of three definitions on dimensions of the family. We find significant differences across the three definitions and by stages of the life cycle, and we discuss implications for our understanding of family structure and functions in the U.S. and elsewhere, and some policy and programmatic consequences.