Prompted by Muslim children reporting their fear of the PREVENT Counter-Terrorism Strategy, this thesis develops a Critical Realist approach to critical discourse analysis (CDA) to describe the violent discourse of 'radicalisation' and 'extremism' (RadEx) from which PREVENT has emerged. RadEx describes the increase in the usage of 'radicalisation' and 'extremism' in British political discourse since the 1970s and how the words became progressively synonymous with violence between 2009 and 2014. It is theorised that RadEx not only suppresses dissent, but also has the capacity to promote violence. The analysis of parliamentary texts shows that RadEx has emerged from earlier colonialist discourses and the loss of parliamentary calculus, a genre of parliamentary discourse that moderated oppressive policy by the threat that it might solicit the emergence of 'radicalisation' and 'extremism'. Aligned with Laclau and Mouffe's socialist strategy, parliamentary calculus led left-wing politicians to embrace opposition and to use parliamentary calculus and the threat of coming to power to moderate the policy of the ruling party. New Labour's aspiration to be in power is shown to have been an abandonment of this previous socialist strategy and to have undermined parliamentary calculus. The discursive change that this precipitates in relation to RadEx is theorised in the semiotic helix. As well as contributing to an understanding of the emergence of RadEx, the semiotic helix also contributes to understanding of discursive change over time more generally. Both Dialectical Critical Realism and metaReality are used to explore how RadEx might be surmounted and it is theorised that the Government's recent expansion of counter-extremism strategy can and should be contested.