This chapter examines how the new notion of vulnerability came to be incorporated in ethical debates in healthcare. It was first introduced in the Belmont Report in 1979. Subsequently it was articulated in international documents of the World Medical Association and UNESCO. It also received more importance, evolving from a moral consideration into a guideline, and ultimately into an ethical principle. However, controversies continue to exist regarding the status of the notion, its content, scope, and implications. Nevertheless, vulnerability is currently considered as an indispensable concept for bioethics.