Since the late 1960s terrorism perpetrated by ETA and the IRA has constantly targeted European liberal democracies such as Spain and the United Kingdom. Both organizations, which are part of what Professor David Rapoport has called the third wave of modern terrorism, have espoused an ethno-nationalist ideology on the basis of which they have justified indiscriminate and intense campaigns of killings for decades. This paper will analyze the motivations of those individuals who at same stage were part of these terrorist organizations, groups which have been responsible for the highest level of deaths in European liberal democracies in the last forty years. Based on extensive in-depth and taped semi-structured interviews of both ETA and IRA activists carried out by the authors, the presentation will analyse the importance that social, psychological, political and other factors had in their decisions to join. The authors will examine how those who joined ETA and the IRA shared some common characteristics while differing in others. Whereas most of those who joined ETA did regard themselves as nationalists when they entered the organization, such a strong feeling was not always present in the men and women who approached the IRA. Both organizations did recruit young activists who shared the common belief that violence was useful and would help them to advance their objectives. The fulfillment of social expectations and the reinforcement of a social identity, together with a marked hatred towards the targets of their violence, were also evident in both groups of terrorists. In short, the paper will look at why individuals joined the most prominent ethno-nationalist terrorist organizations in Europe and why they opted for the use of terrorism.