Since September 11th 2001 (9/11), substantial research has been conducted and published on the phenomenon of Islamist motivated militant terrorism; notably on the contemporary phenomenon of ‘global Jihadism’ and its growing threat to nations and the wider international community. However, little has analysed the different phases of ‘ideological evolution’ and ‘geopolitical transformation’ manifested within the wider phenomenon itself. As a result, Islamist militant terrorist organisations are often lumped together and defined, or identified, as belonging to a single ideological and geopolitical homogenous movement. This study endeavours to highlight that not only is this notion incorrect, but that the phenomenon is threefold consisting of three fundamentally different ideological and geopolitical phases of Islamist militant terrorism and thus categories of Islamist militant terrorist organisations in the Middle East and Southeast Asia. The three phases and categories of Islamist militant terrorist organisations are identified in the study as ‘National’, ‘Regional’ and ‘Global’. Each comprises and represents different ideological contours and geopolitical ambitions. Collectively, these three phases and categories of organisations make up this study’s hypothesis that there have been three ideological and geopolitical sequential shifts in the phenomenon. The study’s sequential shift paradigm tracks the trajectory of the phenomenon beginning with the manifestation of National Islamist militant terrorist organisations, developing into Regional Islamist militant terrorist organisations and, finally, into Global Islamist militant terrorist organisations. The hypothesis is tested through an examination of Islamist militant terrorist organisations in the Middle East and Southeast Asia. Here, the considered organisations are measured against the sequential shift paradigm, which provides key insight into their ideological and geopolitical contours. In turn, this helps to identify and categorise the National, Regional and Global phases and categories of Islamist militant terrorist organisations in the two considered regions and, therefore, further advances support for the study’s hypothesis. The analysis presented in this thesis on the phenomenon of Islamist militant terrorism finds that the study’s hypothesis applies and is analytically accurate in the context of the Middle East. In the context of Southeast Asia the analysis supports that there has been a shift from National to Regional Islamist militant terrorism but, however, that the third and hence Global phase of the phenomenon has thus far, although active in the region, failed to fully manifest. Therefore, the analysis presented in this thesis supports that the phenomenon of Islamist militant terrorism in Southeast Asia has not undergone a decisive ideological evolution or geopolitical transformation into Global Islamist militant terrorism.