This essay considers the frequent and varied uses of 'denial' in modern political discourse, suggests the specific psychoanalytic meanings the term has acquired and asks how useful this Freudian concept may be for historians. It notes the debates among historians over the uses of psychoanalysis, but argues that concepts such as 'denial', 'disavowal', 'splitting' and 'negation' can help us to understand both individual and group behaviour. The authors dwell, especially, on 'disavowal' and argue it can provide a particularly useful basis for exploring how and why states of knowing and not knowing co-exist. Historical examples are utilized to explore these states of mind: most briefly, a fragment from a report about the war criminals, produced by an American psychiatrist at the Nuremberg Trial; at greater length, the political arguments and historical writings of an eighteenth-century slave-owner; and finally, a case in a borough of London in the late-twentieth-century, where the neglect, abuse and murder of a child was shockingly 'missed' by a succession of social agencies and individuals, who had evidence of the violence available to them.