The concept of cumulative extremism has gained significant policy currency in recent years, with some authors seeking to explore and develop conceptual understanding of the phenomenon. With the resurgence of right wing extremist groups – and especially the growth of ‘counter-Jihad’ organisations in the early 21st century – this work has increasingly been employed to give form to explanations linking acts of violent Islamist terrorism and spirals of Islamophobic violence perpetrated by far-right groups against members of the broader Muslim community offline. However, with patchy evidence of increased post-terror attack hate speech online, questions are increasingly being asked about how far orthodox cumulative extremism theory can be applied to the online space. This chapter attempts to address this question, presenting the results of analyses conducted using social media data from both the English Defence League and Britain First in the period immediately before and after the terrorist attacks in Paris in November 2015. In contrast to orthodox scholarship on cumulative extremism, it highlights that, rather than magnifying extremist attitudes and triggering calls to action that result in online attacks, there is comparably little evidence of a change in activity in the posts of either group following the terror attacks. This challenges the ideas of orthodox cumulative extremism theory, suggesting that the perceived threat of radical Islam will not automatically mobilise right wing extremist groups. The reasons for this are discussed at length, alongside the possible policy implications this may have.