Concern for human vulnerability seems to be at the heart of bioethical inquiry, but the concept of vulnerability is under-theorized in the bioethical literature. The aim of this article is to show why bioethics needs an adequately theorized and nuanced conception of vulnerability. We first review approaches to vulnerability in research ethics and public health ethics, and show that the bioethical literature associates vulnerability with risk of harm and exploitation, and limited capacity for autonomy. We identify some of the challenges (...) emerging from this literature: in particular, how to reconcile universal human vulnerability with a context-sensitive analysis of specific kinds and sources of vulnerability; and how to reconcile obligations to protect vulnerable persons with obligations to respect autonomy. We then briefly survey some of the theoretical resources available within the philosophical literature to address these challenges, and to assist in understanding the conceptual connections between vulnerability and related concepts such as harm, exploitation, needs, and autonomy. We also sketch out a taxonomy of sources and kinds of vulnerability. Finally, we consider the implications for policy evaluation of making vulnerability an explicit and central focus of bioethics. Our investigation is in the form of a broad survey motivating a research agenda rather than a detailed analysis. (shrink)