The concepts of sustainability and sustainable development are frequently described as having three main components, sometimes referred to as the three pillars or the triple bottom line: environmental, economic, and social. Because of an historical focus in the sustainability field on correcting environmental problems, much consideration has been given to environmental issues, especially how they interface with economic ones. Frequently mentioned but rarely examined, the social aspects of sustainability have been considered the weakest and least described pillar. After a brief review of existing concepts and theories, this paper uses a case study approach to examine the third pillar more comprehensively and offers social capital as one measure of social sustainability. Specifically, social capital was used to measure the social-environmental interface of communities. The positive correlation between aspects of the built environment, specifically walkability, and social capital suggests that measuring a social aspect of sustainability may be feasible, especially in the context of community development.