The Royal Monmouthshire Royal Engineers (Militia) is the most senior regiment of the British Army Reserve. The regiment was formed in 1539 during the reign of by King Henry VIII, making it the second oldest regiment of the British Army (The Honourable Artillery Company was formed in 1537). The R Mon RE (M) became a militia unit in 1660 and then became a part of the Royal Engineers in 1877.

The Army Reserve is the active-duty volunteer reserve force of the British Army. It is separate from the Regular Reserve whose members are ex-Regular personnel who retain a statutory liability for service. The Army Reserve was known as the Territorial Force from 1908 to 1921, the Territorial Army (TA) from 1921 to 1967, the Territorial and Army Volunteer Reserve (TAVR) from 1967 to 1979, and again the Territorial Army (TA) from 1979 to 2014.