In recent years, there has been an intensification of international extremist violence linked in varying degrees to Internet-facilitated radicalisation. This has related to, among other things, a growth in prevalence of politically violent actors, including far-right and jihadist collectives. Extreme political polarisation, sometimes termed the ‘hyper-tribalism’ of those with violent or extreme views, is to some extent reinforced by these entities’ participation in social media. Radicalisation to terrorism is also arguably facilitated by the architectures of social media platforms, which comprise of personalisation algorithms and the re-mediating functions of ‘likes’, ‘shares’, and ‘re-tweets’ (Sunstein 2009; Pariser 2011; Wood 2017). This chapter reflects on characteristics of social media that can be perceived to encourage violent extremism and terrorism, and legal, security, and technological measures that have been developed internationally to prevent and counter online violent extremist expression. With reference to recent terrorism-related trends, it also highlights a disparity in legal measures to address far-right hate speech (Gelber & McNamara 2016; Davey et al. 2018), relative to thoseused to police and restrict online activity related to jihadism (Conway et al. 2018).