The main aim of this thesis is to introduce a new way of thinking about security within International Relations by developing a model that can be used to explain the relationship between terrorism and organised crime. Referred to as the crime-terror continuum (CTC), the model identifies six major points of convergence between the terrorist and criminal worlds. The crime-terror continuum seeks to move away from the traditional confines of International Relations as encapsulated within realist thought. After providing an overview of the limitations of traditional theories, and a working definition of terrorism and organised crime, this thesis applies an alternative conceptual framework - based on a combination of applicable assumptions about security presented by the Copenhagen School, Ken Booth and Mohammed Ayoob - to an understanding of the threats posed by terrorism and organised crime. It also incorporates the understanding of the contemporary security environment provided by the globalisation and netwar proto-paradigms as a way to go beyond debates about concepts by seeking to understand the operational and organisational dynamics of contemporary security threats. Paying special attention to the argument that non-state actors can be equal to state actors in the security domain, this thesis highlights that competition over state functions and territory continues to play an integral role. The alternative view of security and the CTC are subsequently applied to two case studies: Russian Organised Crime and the Revolutionary Armed Forces of Colombia. Despite illustrating different aspects of the CTC, these case studies highlight the ability of the conceptual framework and the CTC to explain and understand the post-Cold War security environment.