Originally coined in 2006 by prominent far right scholar, Roger Eatwell, cumulative extremism (or ‘the way in which one form of extremism can feed off and magnify other forms [of extremism]’) has become a popular term in the social scientific and policy lexicon—being discussed variously as ‘tit-for-tat radicalisation’ (Jackson, The EDL: Britain’s ‘New Far-right Social Movement. Radicalism and New Media Group, Northampton, 2011), ‘cumulative radicalisation’ (Bartlett and Birdwell, Cumulative Radicalisation between the Far-Right and Islamist Groups in the UK: A Review of Evidence. London: Demos, 2013) and the ‘connectivity between extremisms’ (Ranstorp, Understanding Violent Radicalisation. Abingdon: Routledge, 2010). Despite recent work to refine this concept, however, there have been few attempts to operationalise the term empirically (Pupcenoks and McCabe, Journal of Muslim Minority Affairs 33(2), 171–184, 2013; Busher and Macklin, Behavioural Sciences of Terrorism and Political Aggression, 7(1), 53–68, 2015b)—especially when it comes to the online space. This chapter hopes to remedy this. Using Twitter accounts of key movements within each extremist milieu, it will explore several notable periods of rhetorical escalation and non-escalation by the U.K. far right and some radical Islamist groups online. This will be done in order to increase our understanding of how such movements feed off each other but also to refine the validity and reliability of the concept when describing such interactions between extremist organisations.