This chapter brings a philosophical perspective to the concept of stem cell. Three general questions both clarify the concept of stem cell and emphasize its ambiguities: (1) How should we define stem cells? (2) What makes them different from non-stem cells? (3) What is their ontology? (i.e. what kind of property is “stemness”?) Following this last question, the Chapter distinguishes four conceptions of stem cells and highlights their respective consequences for the cancer stem cell theory. Determining what kind of property stemness is, in what context, is an urgent question, at least for therapeutic strategies against cancers. I hope that this chapter also illustrates how philosophy can be useful to biology. What is a stem cell? The traditional answer to this question is that a stem cell has two properties: the ability to self-renew and the potential of differentiation. This traditional characterization of stem cells raises two questions. First, what do we mean by “ability to self-renew” and “potential of differentiation”? Second, can these two properties distinguish stem cells from non-stem cells? The first question is a problem of definition whereas the second one is a problem of classification. Together, they raise a third question about stem cells: what is their ontology? That is to say, do they belong to a common natural kind? Does the concept of stem cell refer to the cells that belong to a ‘stem cell’ natural kind or does it refer to a reversible and transient cell state? And what difference does that make? This chapter will review the philosophical, theoretical and biological studies that have given insights on these questions. The first part of the Chapter will clarify the notions of self-renewal and differentiation. This will lead to the question “can we (and if so, how) distinguish stem cells from non-stem cells through these two properties?” From that will follow an interrogation on whether stem cells belong to a natural kind. On this issue, biologists and philosophers have framed the following alternative: either the concept of stem cell refers to entities (the cells that belong to the stem cell natural kind) or it refers to a transient and reversible cell state. I will argue that four conceptions of stemness should be distinguished rather than two. Finally, I will develop the case of the cancer stem cell (CSC) theory in order to show why it is crucial to answer the ontological question: some therapies might or might not be efficient depending on what stemness is.