I focus on the set of problems that arise in identifying both the actus reus and (to an even greater extent) the mens rea needed by an abettor before she should be criminally liable for complicity in a crime. No consensus on these issues has emerged in positive law; commentators are enormously dissatisfied with the decisions courts have reached; and critics disagree radically about what reforms should be implemented to rectify this state of affairs. I explicitly deny that I will (...) be able to solve these problems, although I hope at least to identify a central source of the confusion. In my view, the problem results largely from conceptualizing the liability of abettors as derivative. This diagnosis helps us to understand why the problem is likely to remain insoluble in positive law. If the test of an adequate theory consists primarily in its ability to produce results that conform to our moral intuitions about how particular cases should be resolved, no approach that can be implemented in the real world will prove wholly satisfactory. I advance a hypothesis about why failure is inevitable and what should be done in light of this predicament. Legal realities compel us to adopt a position that is suboptimal from a moral point of view. (shrink)