This chapter formulates specific components for a theory of extremist side-switching across hostile ideologies. The key aspect discussed is the way in which ideological components are transmitted across a milieu's boundaries. Six questions are explored: the why, what, how, who, where, and when of side-switching. Furthermore, the chapter includes an assessment of the role of gender among the sample of defectors (almost completely male), as well as the prevalence and impact of side-switching. It is shown that the extreme right appears to be much more open and integrative for defectors from their enemy camps as vice versa. Former enemies, especially from the far left, were able to reach significant positions of power and influence in the far right. They also sometimes managed to add completely new strategies and tactics to the extreme right. Within the far-left and Islamic extremist environments, the influence of defectors is severely limited, as far as it can be seen.