Should a legal plea of self- or third-party defence include an ‘awareness component’ that requires that the actor was aware of the justificatory facts at the time of action? Some theorists argue that in cases of so-called unknowing defence, where an actor in fact averts an otherwise unavoidable danger to himself or another person although unaware at the time of action that this is what he is doing, the objective facts alone should allow a plea of self- or third-party defence. Cases of unknowing defence raise issues that are highly significant to the nature of justification and liability. In this article I reject some common approaches to this issue and I offer an account of why acts of unknowing defence are appropriately subject to criminal liability for the complete offence (e.g. murder).