Moral responsibility, I argue, requires agents to do what is within their abilities to act morally. This means that an agent is to blame just in case his wrongdoing is due to an underperformance, that is, to a failure to do what he can to act morally. I defend this account by considering a skeptical argument about responsibility put forth by Gideon Rosen and by Michael Zimmerman. I explain why the epistemic condition they endorse is inadequate and why my alternative epistemic condition, which directly follows from my general condition on culpability, should be preferred. I then defend my view against potential criticisms.