Several investigations of American evangelicalism have been conducted in recent decades, yet they conceptualize evangelicalism in different ways. It is not surprising, therefore, that different profiles of the evangelical movement and its adherents emerge from these studies. This research reviews major studies on the subject undertaken since 1976, when evangelicals first attracted national attention. Using data collected in a single data set—the 1998 General Social Survey—we show how measurement strategies employed in different studies yield drastically different pictures of evangelicalism. Conservative measures indicate that only one in 20 Americans is evangelical while one in two is evangelical according to a combination of more expansive measures. The demographic, cultural, and religious characteristics of evangelicals, as well as theories about them, hinge upon how the population is defined.