Scholars have long recognized that the boundaries of family membership and definitions of family relationships are socially constructed. The social construction of family membership, and the accompanying ambiguity surrounding family language and labels, particularly in complex families who have experienced divorce, remarriage, and other structural transitions, creates obstacles for recruiting study participants and for collecting, analyzing, and interpreting data. In this paper, we explore how the increasing complexity of family structure and family membership can pose pragmatic challenges for researchers. Using our own work as examples, as well as the research of others, we share methodological approaches to addressing these challenges within both qualitative and quantitative research designs. We argue that giving primacy to respondents’ relational definitions changes how researchers approach their projects, stimulates innovative theoretical thinking, and advances understanding of how individuals and families construct their social worlds.