The term “radicalisation” was initially used in medicine and was later adopted to describe a political attitude in the late 1790s. In general, it can be said that the action or process of making someone become more radical or extreme in their political or religious beliefs is called radicalisation. There are two types of radicalisations, namely behavioural and cognitive radicalisation, and in both types, various constitutive elements of radicalisation exist. This chapter will focus on how an individual becomes a terrorist, highlighting the psychological process, vulnerabilities, and strategies. Numerous models have been proposed to explain how individuals become radicalised and ultimately engage in terrorism. Various models and theories of radicalisation have been described in the chapter. The long-term survival of terrorist organisations depends on their ability to attract new members and sustain a continuous recruitment cycle, and recruitment patterns were explained. Radicalisation mechanisms describe how and why individuals or groups shift from nonviolence to political violence. These processes include specific psychological and sociological phenomena, which, while relevant to many, can manifest differently in different cases. Mechanisms for radicalisation have been described, and major points for future studies have been mentioned.