When we think of private foundations, names such as Gates, Ford, and Rockefeller come first to mind. But those billion-dollar foundations represent a very small segment of the philanthropic sector. Of the 72,000 foundations in the United States, fewer than 50 have endowments of more than $1 billion. The vast majority, more than 90 percent, have endowments of less than $10 million. Most of these organizations are family foundations.