 For most newcomers to the problem of free will, it will seem obvious that an action is up to an agent only if she had the freedom to do otherwise. But what does this freedom come to? The freedom to do otherwise is clearly a modal property of agents, but it is controversial just what species of modality is at stake. It must be more than mere possibility: to have the freedom to do otherwise consists in more than the mere possibility of something else’s happening. A more plausible and widely endorsed understanding claims the relevant modality is ability or power (Locke 1690 [1975], II.xx; Reid 1788 [1969], II.i–ii; D. Locke 1973; Clarke 2009; Vihvelin 2013). But abilities themselves seem to come in different varieties (Lewis 1976; Horgan 1979; van Inwagen 1983, ch. 1; Mele 2003; Clarke 2009; Vihvelin 2013, ch. 1; Franklin 2015; Cyr and Swenson 2019; Hofmann 2022; Whittle 2022), so a claim that an agent has ‘the ability to do otherwise’ is potentially ambiguous or indeterminate; in philosophical discussion, the sense of ability appealed to needs to be spelled out. A satisfactory account of the freedom to do otherwise owes us both an account of the kind of ability in terms of which the freedom to do otherwise is analyzed, and an argument for why this kind of ability (as opposed to some other species) is the one constitutive of the freedom to do otherwise. As we will see, philosophers sometimes leave this second debt unpaid.