One way to look at families is based on structural features. Families contain varying numbers of persons who are related in particular ways, including such persons as mothers, fathers, and children. This view may be extended to include grandparents, in-laws, step-relations, and perhaps even former relatives. Structural definitions of family focus on the composition of its membership. They may indicate that family members are related by blood, marriage, or some other legal bond such as adoption. Sharing a household may be another structural feature. With a structural definition, the theorist is able to determine which kinds of social groups do not qualify as families and which individuals are in a particular family.

Structural definitions of family also attend to the types of relationships that create social bonds between members. Important bonds are created by communication, power, and affection, as well as the daily work and leisure performed by family members. Scientists can observe how patterns of social interaction among the members are structured, and they can specify the various rules or principles that families use to organize their activities. Families may be structured by such characteristics as gender, age, and generation, as well as their connections to the outside world. These structures also are useful for distinguishing families from other kinds of social groups and organizations.