 This article falls into three parts. The first part, Sections 1 and 2, states a framework for discussing philosophical theories of ability. Section 1 will say more about the distinction between abilities and other powers of agents and objects. Section 2 will make some formal distinctions that are helpful for framing any theory of ability. The second part, Sections 3–5, surveys theories of ability that have been defended in the philosophical literature. Section 3 concerns the most prominent kind of theory, on which abilities are to be understood in terms of a hypothetical relating an agent’s actions to her volitions. Section 4 concerns theories that are not hypothetical in this way, but that nonetheless retain the basic reductive orientation of hypothetical theories. Section 5 then discusses various alternative theories of ability that have been proposed in the recent literature. The third part, Section 6, turns to the relationship between a theory of ability and the free will debates. Such debates often involve claims about agents’ abilities, and many have hoped that getting clearer on abilities themselves could resolve, or at least shed light on, such debates. The aim of this last section will be to assess whether these hopes are reasonable ones.