Family structure refers to relationships at the juncture of biological relatedness, marriage, comparable legal relationships including adoption, and living arrangements. Census data provides information on family structure (e.g., US Census Bureau, 2022). Definitions of family can differ between countries, and even from census to census within countries. This approach seeks to categorize, quantify, and measure existing family structures and make comparisons over time. We introduce key concepts and definitions critical for our understanding of families and family structures mainly within a Western and US context. Then, we will discuss family structure referring to biological evolutionary, historical, sociological, and psychological theory and research.