This dissertation studies the use of terrorism in domestic Portugal throughout a significant part of the twentieth century, from the establishment of the Estado Novo to the formation and consolidation of democracy. Albeit with varying levels of impact in terms of the stigma inflicted upon the populace, and varying degrees of relevance in terms of capacity to mobilise for change or to achieve compliance, the argument to be discussed is that the phenomenon of terrorism has prevailed throughout this period of Portuguese history and has been present at the enfolding of crucial political transitions. Following a conceptual discussion on terrorism, an attempt is made to produce a definition based upon its behavioural essentials, pandemic to any social linkage in whatever political context. Since different stages of Portuguese history involve different actors engaged in terrorist practices, a review of theories on terrorism from which to profit from throughout the analysis is presented. The Estado Novo is analysed as a regime of intimidation, partially sustained by its terrorist practices and to some extent accountable for the on going disguised anaemia of the strong Portuguese spirit. The illegitimate and abnormal use of force by the Estado Novo raises problems regarding the classification of the organisations that chose the path of violent opposition to the regime, difficulties that are overcome by the definition of terrorism developed in this work. An attempt is made to describe these organizations, to clarify their origins, ideology, internal backup and external support, as well as understand their modus operandi, evaluate their relevance and explain their disappearance. The practice of terrorism is also observed in the aftermath of the 25 April during the period of establishment and consolidation of democracy in Portugal. Emerging both at the left and the right of the political spectrum, this activity is firstly analysed as a reflection of the clash between revolutionary change and counter-revolution, and then is studied in the light of a democratic norm. An attempt is made to describe the major terrorist organizations of both the radical right and the extreme left.  The processes of controlling and pacifying Portuguese society are henceforth presented, clarifying the path that transformed Portugal into a society devoid of the terrorist phenomena.