My thesis seeks to resolve key areas of debate regarding the nature of provocation as a defence in English Law, by reference to both historical and philosophical analysis. Academic commentators on the doctrine disagree on whether it should be seen as essentially an excusing condition alone, such as insanity, or should be understood as an excuse involving some element of "partial [moral] justification", such as duress. I seek to resolve this debate by considering at a deeper philosophical level the nature of anger and action in anger. I argue that English Law has historically operated with two equally plausible conceptions of anger, anger as righteous indignation and anger as a loss of self-control. The former was the conception drawn on in the development of the early modern law, and the latter is the conception drawn on in the development of the modern law. I go on to argue that controversy, referred to above, over the nature of provocation as a defence, can be only resolved by making clear the distinction between the two kinds of anger. Action in anger conceived as a loss of self-control bears more of a family resemblance to defences such as (temporary) insanity. Action in anger conceived as righteous indignation bears more of a family resemblance to defences such as duress. I argue that whichever conception of anger is in issue, however, action in anger can be open to moral criticism, because all kinds of action in anger are based on at least a judgment of moral wrongdoing, that may be subject of such criticism. I argue that the law may thus legitimately require at least some moral justification for action in anger before allowing a defence of provocation to be pleaded successfully.