Command and Control (C2) systems form an integral part of military defence in supporting operational military decision-making. C2 systems are complex socio-technical systems made up of physical hardware, software, data, Operators, Maintainers, processes and the workplace. One of the complexities is multiple Operators performing different roles simultaneously, under high workload, making rapid decisions to work towards a common goal. This paper out-lines an ongoing project to replace an existing C2 system. A Human Reliability Assessment (HRA) is being conducted with an approach which incorporates a more holistic assessment of the human contribution within ‘the system’. Identifying not only opportunities to identify and mitigate potential human error by influencing design but also looking at the wider interactions within the system and identification of emergent properties which may result from more complex interactions within the system.