The idea of social capital has had a powerful impact on the study of politics, policy, and social science at large. Much of what we know about the causes and effects of social capital, however, is limited by the nature of data used regularly by scholars working in this area. Current data sets allow researchers to study changes in social capital over time at the national level and static differences in the distribution of social capital across the states. The inability of scholars to know how social capital varies over time and across space limits the kinds of questions that can be asked. In this article, we use sample data of more than 20,000 individuals conducted biannually in the continuous 48 states by the marketing research firm MediaMark Research, Inc. to create a unique measure of social capital that varies across time and space. These data are available at the state level from 1986 through 2004.