The Thames Estuary is the gateway into London that had to be defended against seaborne invasion. Through proximity to the Continent, these waters were a likely passageway for those intent upon seaborne raids or invasion, necessitating the need for a powerful naval force to be on hand when threatened. The first fortifications date back to Roman times. To support the British navy in these waters, four of the nation’s royal dockyards – Chatham, Deptford, Sheerness and Woolwich – were clustered along the Thames Estuary or close by on the Medway from the sixteenth and seventeenth centuries for the commissioning, refitting and repair of warships. As well as being of importance for the defence of the country, the Thames Estuary fulfilled another role: that of underpinning naval activities designed to support British tactical and strategic operations in more distant parts of the world. Close to the mouth of the Thames, and near the point of confluence with the Medway, was the Nore, a key naval anchorage where newly commissioned warship assembled, taking on crews and receiving final instructions before joining the active seagoing fleet. In the twentieth century, additional defences against attack by submarine or from the air were established, and gunpowder factories sited along the estuary. This book will be of interest to all those who would like to know more about the remarkable military history of the Thames Estuary over the last 2, 000 years.