There are currently 548 declared military facilities in the United Kingdom, located on 372,000 hectares of military-owned or used land. Collectively known as the defence estate, this land is used for defence and training, and constitutes approximately 1.5% of the UK surface area. The research presented here interprets this landscape and its accompanying airspaces, infrastructures and processes as a spatial phenomenon, one which is in an almost constant state of flux. This thesis is, therefore, a study of militarised space in the UK as defined by recent developments in technology, mobility and communication. It analyses the processes by which land and space become militarised within different environments and the residual effects of this on the wider fabric of civil society. This thesis addresses issues of land appropriation, weapons testing, airspace design and notions of temporary, flexible, invisible boundaries. A multidisciplinary approach is adopted to analyse the histories, geographies and technologies evident at three case study sites. These are MoD Shoeburyness (Essex), Salisbury Plain Training Area (Wiltshire) and the city of Portsmouth (Hampshire). The first outcome of this research is a spatial interpretation of the defence estate, its transformation over the 20th century and its fluctuating control of the British landscape and skies. The second outcome is an analysis of the current military environment and its use of complex assemblages of land, sea, infrastructure and airspace to contain military activities. These localised, three-dimensional forms are not only becoming more refined to accommodate new weapons and technologies but they are also increasingly connected to each other. The third outcome is a speculative interpretation of the defence estate as a complex, connected totality, a parallel landscape of military spaces, activities and processes. Together, these outcomes demonstrate that the Armed Services of the UK preside over an increasingly complex and interconnected environment. They continue to engage with UK territory and space in unique and diverse ways but are increasingly influenced by external forces such as the commercial and civil sectors, public interest pressure groups and the conflicted governance of the state.