This paper starts with a discussion of definitions of social capital, then turns to issues in measurement, and finally, presents some evidence on the consequences of social capital. In the last five years, I have been working exclusively on some specific and perhaps unique problems about social capital in the United States, so all of my examples are going to be drawn from the United States experience. I don’t want to be interpreted as saying these trends are common to all OECD countries. It is just that the United States has been the main focus of my research for the past five years.