Introduction: While Civil Military Relationships (CMRel) have occurred for hundreds of years, they have changed dramatically in the last 30. First, there was a period of increasing cooperation driven by United Nations interventions in the 1980s and 90s. Then a contraction, as the impact of blurred lines between humanitarians and militaries became more obvious during the Balkans, Iraq and Afghanistan. The last decade in particular has seen increasing securitisation of health, driven on the one hand by the global war on terror and counter-terrorism legislation, and on the other by fear about increasing globalisation and the subsequent risks of transnational infectious diseases. Current guidance is driven by the civilian desire to re-establish clarity about what is humanitarian and what is not, but the lines between humanitarianism, development and peacebuilding have all but vanished-with the CIA using polio vaccination programmes to cover the hunt for Bin Laden and the ICRC supporting the Pakistan government in the routine delivery of emergency care. CMRel are likely to happen more often. While traditional Disaster Relief CMRel have not gone away, as the impacts of climate change and increasing urbanisation mean that populations are increasingly vulnerable to catastrophe, CMRel driven by transnational threats and global competition mean that they will also happen in a range of new contexts. Methods: This is a mixed methods study. The literature was scrutinised through a combination of narrative, hermaneutic and systematised short-cut reviews. Data was also collected in the form of semi-structured interviews with 37 experts experienced in the civil-military arena, be it in Disaster Relief, Conflict, or other settings. Data was also captured using questionnaires, including the first analysis of CMRel in the field-on the United Nations Mission in South Sudan-and a comparison of various CMRel scenarios in order to validate a proposed typology. Finally, Field notes were taken according to ethnographic guidelines on a range of operational deployments and at relevant conferences, meetings and courses worldwide over the course of the PhD (2018-2022). Content analysis of interview and text data was undertaken using nVIVO software (QSR International) and themes identified, which respondents felt described or modified CMRel. There was some limited quantitative analysis to assist in triangulation of the themes (using word frequency counts) and validation of the typology. Results: The literature reviews found that International Humanitarian Law specifically "made of the military medical professional a special case, a neutral resource and Humanitarian asset, who served all combatants" (Koch, 2006). However, this concept appears to have been largely dismissed outside the warfighting context. Certainly, the fact that Healthcare itself is a natural civil-military partnership in many parts of the world barely features in either the civilian or military discourse. Warfighting and Disaster Relief Operations (DRO) CMRel are clearly defined, and the civilian and military guidance is generally consistent. In DRO the key principles are of 'Last Resort' use of the military, ensuring that the overwhelming onus of the response is civilian owned and led. The military contribution should be time-limited, designed to fill critical Humanitarian gaps and distinct from the civilian element. In Complex Humanitarian Emergencies the picture is less clear. While guidance exists, there is still a divergence between the military position where CMRel support the mission, and the IASC guidelines which focus on distinction and distance, to protect the principled approach of Humanitarian actors. In peacebuilding/development there is little guidance. A typology is derived which helps to understand the wide range of CMRel that can occur, and the confusing nomenclature that results. It postulates that there are three main types, depending on the primary driver: Military-led CMRel, Civilian-led and Health driven. Competitive can also occur but is infrequent. Each type has different characteristics in terms of the contexts where they are most likely to occur and their impact. The types can be mapped against different scenarios, using three axes: level of engagement (strategic to tactical), context (stable to highly unstable), and health motivation (humanitarian or development). This demonstrates that in some contexts there may be the potential for several different types of CMRel to occur, and two in particular might be deemed higher risk than others. One (at the tactical level, in conflict) is well recognised from the literature review; one (Peacetime Military Engagement in stable contexts) is not. The typology was validated through a series of scenarios at different points on the map. The consensus amongst civilians and military respondents was marked, and both proposed areas of high risk were identified, validating the typology. CMRel themes arising from conflict and non-conflict contexts were evaluated through the interviews and also case studies based on field notes. While many themes are well recognised, many are new: in particular the clear signal that Medical CMRel are different from those undertaken by the wider military. There was very little overlap between the themes that arose from conflict and non-conflict settings, which is not previously described. An earlier framework for CMRel is modified to explain the observed themes. The new CMRel Trinity proposes that they are based on three pillars: Physical effectiveness, Coordination and conceptual development, and a Moral Component. This framework accommodates all the themes and explains the differences observed. This is followed by an exploration of the role of militaries in diplomacy and health system capacity building (Defence Healthcare Engagement, DHE). Synthesis of the preceding findings results in a framework for effective DHE. The cases for and against closer medical CMRel are explored and it is concluded that Medical CMRel should be closer. Barriers to this are outlined and a way forward proposed. Conclusions: This thesis presents a new way of looking at Medical CMRel. It identifies areas of consensus, debate and contradiction in the literature, as well as areas that are poorly described. It derives a typology that is able to encompass all Medical CMRel, depending on the main drivers and contexts for the relationship. The typology is then validated. The marked differences in the themes important to CMRel, expressed in interviews and observed in the case studies, are explained by a novel model whereby CMRel comprise Physical, Coordination/Conceptual and Moral components. A neglected area in the literature, Defence Engagement, is explored and understood. Together this thesis allows an understanding of CMRel, what makes them harmful or beneficial, and how they can be optimised in future that was previously missing from the literature. In so doing 15 papers and one book chapter have been published to date. The work also forms the basis of the UK Defence Medical Services' Defence Engagement Strategy.