Fanfiction has been hailed as 'the democratic genre' (Pugh 2000), its proponents celebrated as 'textual poachers' (Jenkins 1994) who radically disrupt but also reinvigorate canonical texts. Over time, aspects of plotting and characterisation introduced by fanfic writers may become 'fanonical', accepted by the fans as being just as intrinsic to the storyworld as any aspect of the 'original' or 'source' text. Focusing on literary fanfiction online, this article will trace the emergence of 'fanons' within specific fanfic communities, analysing the extent to which they either draw on, or depart from, the 'source' texts. Alongside this, the article also explores how far fanons are either openly or covertly self-policing, and address the fundamental tensions between fidelity and deviance, dependence and freedom that underlie the whole fanfiction phenomenon.