Paper presented at the 8th Biennial Conference of the International Society for Ecological Economics. One of the important new trends in population-environment studies has been the consideration of households as the unit of analysis. Focusing on households is important because much environmental significant activity is “consumed” at the household level, e.g., personal transport and residential energy use. Furthermore, in many countries in which population has remained constant or declined, the number of households has increased and household composition (e.g., number of members, age and gender of head) has changed substantially. This paper examines US household data (from the Residential Energy Use Consumption Survey) and employs standard techniques from the demographic literature to determine to what extent compositional changes in the nature of households can explain changes in per capita environmental impact (e.g., miles driven and residential energy use) both over time and across regions.