Existing historiography on the Royal Marines emphasises the institution’s early roles in the Age of Sail or more recent history as Commandos. While historically the Marines were formed from early army regiments, in 1755 the Marines were permanently established under the Admiralty, retaining much of its organisational structure and traditions from the army. Honoured as a Royal Regiment in 1802, they were left largely free to develop an independent organisational identity in line with established duties at sea and on shore. The Royal Marines, however, did not share an equal footing with their peer services. As a sub-organisation of the Royal Navy, Marines were sensitive to their inferior relative status with the army and navy and a common lament of Marines was a lack of public recognition. This was especially true for Marine officers who struggled to fit in to a rigid hierarchical military social structure. By the mid-nineteenth century, changes in warfare and technology eroded once established roles and missions of the Royal Marines. Royal Marine identity became increasingly in conflict with the Royal Navy and impeded a clear solution for a new operational mission and purpose. Absorption by the army, outright elimination, or forced restructuring of their organisation threatened the extinction of the Royal Marines. As the nineteenth century progressed, the Marines wrestled over concerns of redundancy and their officers as superfluous aboard ship. These portrayals contrasted with an often-blameless record of service and reputed bravery and efficiency. Opportunities to reorganise and reform at the conclusion of the First World War were mismanaged, necessitating a complete transformation in the mid-twentieth century. Challenging recent historiography on the organisational history and identity of the Royal Marines, this thesis demonstrates how the existential struggle of the Royal Marines was rooted in a series of crises and events between 1827 and 1927.