This chapter discusses criminal liability for omissions. The criminal law draws a basic distinction between things done and things not done. Its default rule is that one is not accountable for failing to prevent something that it would be a crime to bring about by a positive act. There are, of course, exceptions to the default rule, in as much as the law often imposes distinct duties to intervene and prevent harm. However, the concern in this chapter is with why the default rule itself should exist. One reason is that not-doings are typically less culpable, and the law has good reasons to acknowledge this by means of its default rule requiring a positive act. More importantly, though, the doing/not-doing distinction matters for reasons of ascriptive responsibility. Distinct duties are, first and foremost, conduits to holding the defendant accountable for an event or outcome. Absent such a duty, a not-doer is prima facie not accountable for the consequences of her deed: whereas a doer is. The chapter looks first at the nature of not-doings and omissions generally; considers whether not-doings really are less culpable; then investigates the argument from ascriptive responsibility for differentiating their treatment.