Counterfactuals about what a patient would consent to, if he were able to consent, are often cited as justifications, or partial justifications, for acts of euthanasia. In virtue of this fact, they deserve special scrutiny by moral philosophers. In Chapter I, I examine terminology that is essential to further understanding the relationship between euthanasia and counterfactual consent. I propose a definition of 'euthanasia', an analysis of 'consent', and I present a brief description of counterfactuals. In Chapter II, I consider two questions. The first is, "When it is appropriate to invoke counterfactual consent in an attempt to justify an act of euthanasia?" By making use of an improved version of the voluntary, nonvoluntary, and involuntary distinction among acts of euthanasia, I am able to determine when it is appropriate to cite counterfactuals about consent in an attempt to justify an act of euthanasia. The second is, "to what end is counterfactual consent used?" I contend that counterfactual consent does morally justify some acts of euthanasia, and defend an argument for this claim. Finally, I look at the role of counterfactual consent as a possible legal justification for acts of euthanasia. In Chapter III, I use possible world semantics to analyze counterfactual consent. Traditional counterfactuals are determined to be true if in the closest world at which their antecedent is true, their consequent is also true. Counterfactuals about consent have a less straightforward reading. I consider and reject several possible ways of reading counterfactuals about consent, before settling on the correct reading of counterfactuals about consent. In Chapter IV, I consider evidence for the truth of claims about counterfactual consent. I consider and reject the claim that no counterfactual is either true or false. I examine both Living Wills and the practice of surrogacy, neither of which offers sufficient evidence for the truth of claims about counterfactual consent. In Chapter V, I contrast counterfactual consent with actual consent. I review and refute the arguments for the claim that actual consent is preferable to counterfactual consent. I conclude by presenting a principle about the relationship between actual and counterfactual consent.