This thesis aims to solve two metaphysical puzzles about ability. The first belongs to the free will debate: is determinism compatible with the ability to do otherwise? The second pertains to time travel: what things can a backward time traveler do in the past? By introducing two principles about an agent’s ability regarding the past, I defend the following modal principle about ability: what an agent can do must be compossible with the past. This principle generates solutions to the two puzzles.