An object's disposition to A in circumstances C is masked if circumstances C obtain without the object Aing. This paper explores an analogous sense in which abilities can be masked, and it uses the results of this exploration to motivate an analysis of agents' abilities in terms of dispositions. This analysis is then shown to provide the resources to defend a version of the Principle of Alternate Possibilities against Frankfurt-style counterexamples. Although this principle is often taken to be congenial to (...) incompatibilism concerning free action and determinism, the paper concludes by using the dispositional analysis of abilities to argue for compatibilism, and to show why the 'master argument' for incompatibilism is unsound. (shrink)