Researching terrorism involves not only a broad study of their activities and motivations, but also involves the collection and interpretation of data concerning the members of terrorist organizations. The current study examined quantitative and qualitative data on over 600 individuals who became members of the ETA terrorist organization between the early 1970’s and the late 1990’s. The goal of the research was to examine the changing sociological profile of these militants over the course of the study period. Data were collected from the judicial summaries and proceedings from 1977 to 1998 of a national court in Spain with exclusive jurisdiction over terrorist crimes. Oral testimonies were also taken from semi-structured, personal interviews with former ETA militants to assist in the interpretation of the quantitative data. The findings indicate that the sociological profile of ETA militants in terms of sex, age, and marital status when they joined the organization is consistent with existing knowledge about members of other terrorist organizations. However, the current findings indicated differences in occupation and social class from other terrorist organizations, perhaps revealing that members of the ETA were not homogeneously recruited. Finally, a surprising inversion in the sociological profile of ETA militants occurred over time, revealing the degree to which membership traits may vary over the lifecycle of a terrorist organization. Tables, notes