In this thesis I contribute to the growing literature on the structure of covert networks by exploring the organisation and functioning of two new groups. (1) The Right Club, a Right-wing, Pro-German group active in the UK at the outbreak of World War Two, and (2) The leadership group of the Provisional Irish Republican Army (PIRA) between 1969 and 1986. Specifically, I focus upon the formation of these groups, and how, and indeed if, they maintained covertness in practice. Whilst there has been a wealth of research in this area, many studies simply assume covertness and its impact upon structure due to the illegal nature of their case studies. In this thesis I develop a more nuanced concept of covertness, and a more detailed analysis of the myriad factors which affect the structure of a clandestine group. I employ a mixed methods approach combining Social Network Analysis with qualitative inquiry of the environment and processes which influence the functioning of each group. The qualitative analysis, which was guided by factors identified in the existing covert networks literature, in the Social Movements literature, and by dynamics noted in work on the Sociology of Secrecy, is used to explore and explain the sociometric findings. This provides a more in-depth, more sociological understanding of clandestine organisation than that which currently exists in this field of research. However, more and varied case studies analysed in this way are also necessary if we are to improve our understanding of the structure and functioning of covert groups. With this knowledge more sensitive and successful deradicalisation and/or destabilisation techniques can be crafted.