The Militia of Great Britain were the principal military reserve forces of the Kingdom of Great Britain during the 18th century.
For the period following the creation of the United Kingdom of Great Britain and Ireland in 1801, see Militia (United Kingdom).
Following the restoration of Charles II in 1660, parliament passed several acts empowering the Lord Lieutenant of each county to appoint officers and raise men for a militia force. Although the king commanded the forces, they were not centrally funded. The burden of supplying men and equipment fell on property owners, in proportion to their income from land or their property value. The militia could be called out for local police actions, to keep the peace, and in the event of a national emergency. It played a role in coastal defence during the second and third Anglo-Dutch Wars between 1665 and 1674, and contributed to the defeat of the Duke of Monmouth in 1685.
The militia's usefulness as a military force, never great, declined thereafter, until by the middle of the 18th century it required a major overhaul. This was achieved by the Militia Acts 1757-62, passed by parliament as a response to the threat of a French invasion during the Seven Years' War. Responsibility for raising and organising the force remained at county level, but funding was provided by central government. Officers were to be appointed from among the property-owning class. Men were to be chosen by ballot among the able-bodied men of the parish between the ages of 18 and 50, and would serve for three years (soon extended to five). If they wished not to serve, they could either provide a substitute or pay a £10 fine.
There was considerable opposition to the reforms, both in parliament and in the country at large. Riots occurred in Yorkshire, Lincolnshire and elsewhere in 1757. These stemmed chiefly from an ill-informed fear that conscription and compulsory foreign service were being covertly introduced. In fact, the acts, which applied in England and Wales only, restricted service to the territory of Great Britain. However some militia regiments did volunteer for service in Ireland during the Irish Rebellion of 1798.