This introduction describes what Phase I trials are and what is expected of healthy people who participate. It also details who typically enrolls in these trials, mapping out the demographic characteristics of healthy volunteers and explaining the seriality of their enrollment in US trials. This background information informs the argument of the book—that US Phase I trials are fundamentally built upon and shaped by social inequalities and that these social inequalities generate risk to study participants as well as invalid study results that may threaten public health. It introduces the two organizing concepts of the book—imbricated stigma and the healthy volunteer as a model organism—and concludes with an overview of the book itself.