Militia
Scottish militia: In the late 17th century, numerous individuals in the Kingdom of Scotland (then in a personal union with the Kingdom of England) called for the resurrection of a Scottish militia, with the understated aim of protecting the rights of Scots in Great Britain. After Scotland became part of the Kingdom of Great Britain, the Militia Act 1757 did not apply there. The traditional Scottish militia system continued, with only certain settlements in Scotland playing host to a militia regiment. This was viewed with resentment among some in Scotland, and the Militia Club was formed to promote the raising of a Scottish militia. The Militia Club, along with several other Scottish gentlemen's clubs became the crucible of the Scottish Enlightenment. The Militia Act 1797 empowered Scottish Lord Lieutenants to raise and command militia regiments in each of the "Counties, Stewartries, Cities, and Places" under their jurisdiction.

19th century: Although muster rolls were prepared as late as 1820, the element of compulsion was abandoned, and the militia transformed into a volunteer force, revived by the Militia Act 1852. It was intended to be seen as an alternative to the regular army. Men would volunteer and undertake basic training for several months at an army depot. Thereafter, they would return to civilian life, but report for regular periods of military training (usually on the weapons ranges) and an annual two-week training camp. In return, they would receive military pay and a financial retainer, a useful addition to their civilian wage. Of course, many saw the annual camp as the equivalent of a paid holiday. The militia thus appealed to agricultural labourers, colliers and the like, men in casual occupations, who could leave their civilian job and pick it up again. Until 1852 the militia were an entirely infantry force, but from that year a number of county infantry regiments were converted to artillery and new ones raised. In 1877 the militia of Anglesey and Monmouthshire were converted to engineers. Under the reforms, introduced by Secretary of State for War Hugh Childers in 1881, the remaining militia infantry regiments were re-designated as numbered battalions of regiments of the line, ranking after the two regular battalions. Typically, an English, Welsh or Scottish regiment would have two militia battalions (the 3rd and 4th) and Irish regiments three (numbered 3rd–5th).
The militia must not be confused with the volunteer units created in a wave of enthusiasm in the second half of the nineteenth century. In contrast with the Volunteer Force, and the similar Yeomanry Cavalry, they were considered rather plebeian.

The Special Reserve: The militia was transformed into the Special Reserve by the military reforms of Haldane in the reforming post 1906 Liberal government. In 1908 the militia infantry battalions were redesignated as "reserve" and a number were amalgamated or disbanded. Numbered Territorial Force battalions, ranking after the Special Reserve, were formed from the volunteer units at the same time. Altogether, 101 infantry battalions, 33 artillery regiments and two engineer regiments of special reservists were formed. Upon mobilisation, the special reserve units would be formed at the depot and continue training while guarding vulnerable points in Britain. The special reserve units remained in Britain throughout the First World War, but their rank and file did not, since the object of the special reserve was to supply drafts of replacements for the overseas units of the regiment. The original militiamen soon disappeared, and the battalions simply became training units. The Special Reserve reverted to its militia designation in 1921, then to Supplementary Reserve in 1924, though the units were effectively placed in "suspended animation" until disbanded in 1953.

The militiamen: The name was briefly revived in the Military Training Act 1939, in the aftermath of the Munich Crisis. Leslie Hore-Belisha, Secretary of State for War, wished to introduce a limited form of conscription, not known in peacetime Britain since the militia of the early 19th century and previously. It was thought that calling the conscripts 'militiamen' would make this more acceptable, as it would render them distinct from the rest of the army. Only single men aged 20 up to the day before their 22nd birthday were to be conscripted, for six months full-time training before discharge into the reserve (with a free suit of civilian clothing). Although the first intake was called up in late July 1939, the declaration of war on 3 September entailed implementation of full-time conscription for all men aged 18–41, superseding the militia, never to be revived.

Modern survivals: Three units still maintain their militia designation in the British Army. These are the Royal Monmouthshire Royal Engineers (formed in 1539), the Jersey Field Squadron (The Royal Militia Island of Jersey) (formed in 1337), and the Royal Alderney Militia (created in the 13th century and reformed in 1984). Additionally, the Atholl Highlanders are a ceremonial infantry militia maintained by the Duke of Atholl—they are the only legal private army in Europe.

Other British militias: Various other part-time, home defence organisations have been raised during times of crisis or perceived threat, although without the word "militia" in their title. These have included:

Volunteer Corps, part of the British anti-invasion preparations of 1803–1805
Yeomanry, volunteer cavalry initially raised in the Napoleonic Wars
Volunteer Force, from 1857 to 1908
Volunteer Training Corps, 1914 to 1918
National Defence Companies, 1936 to 1939
Home Guard, initially Local Defence Volunteers, 1940 to 1944 and 1951 to 1957
Ulster Defence Regiment, 1970 to 1992
Home Service Force, 1982 to 1992

The Troubles and Irish War of Independence: The various non-state paramilitary groups involved in the 20th-century conflicts in Northern Ireland and the island of Ireland, such as the various Irish Republican Army groups and loyalist paramilitaries, could also be described as militias and are occasionally referred to as such.
The Ulster Defence Regiment (UDR) was a locally raised professional militia instituted by an Act of Parliament in December 1969, becoming operational on 1 April 1970. Created as a non-partisan force to defend Northern Ireland "against armed attack or sabotage", it eventually peaked at 11 battalions with 7,559 men and women. 197 soldiers of the UDR were killed as active servicemen, with a further 61 killed after leaving the regiment, mostly by the Provisional Irish Republican Army. As a result of defence cuts it was eventually reduced to 7 battalions before being amalgamated with the Royal Irish Rangers in 1992 to form the "Home Service Battalions" of the Royal Irish Regiment.

United States: The history of militia in the United States dates from the colonial era, such as in the American Revolutionary War. Based on the English system, colonial militias were drawn from the body of adult male citizens of a community, town, or local region. Because there was no standing English Army before the English Civil War, and subsequently the English Army and later the British Army had few regulars garrisoning North America, colonial militia served a vital role in local conflicts, particularly in the French and Indian Wars. Before shooting began in the American War of Independence, American revolutionaries took control of the militia system, reinvigorating training and excluding men with Loyalist inclinations. Regulation of the militia was codified by the Second Continental Congress with the Articles of Confederation. The revolutionaries also created a full-time regular army—the Continental Army—but, because of manpower shortages, the militia provided short-term support to the regulars in the field throughout the war.
In colonial era Anglo-American usage, militia service was distinguished from military service in that the latter was normally a commitment for a fixed period of time of at least a year, for a salary, whereas militia was only to meet a threat, or prepare to meet a threat, for periods of time expected to be short. Militia persons were normally expected to provide their own weapons, equipment, or supplies, although they may later be compensated for losses or expenditures. A related concept is the jury, which can be regarded as a specialized form of militia convened to render a verdict in a court proceeding (known as a petit jury or trial jury) or to investigate a public matter and render a presentment or indictment (grand jury).
With the Constitutional Convention of 1787 and Article 1 Section 8 of the United States Constitution, control of the army and the power to direct the militia of the states was concurrently delegated to the federal Congress. The Militia Clauses gave Congress authority for "organizing, arming, and disciplining" the militia, and "governing such Part of them as may be employed in the Service of the United States", and the States retained authority to appoint officers and to impose the training specified by Congress. Proponents describe a key element in the concept of "militia" was that to be "genuine" it not be a "select militia", composed of an unrepresentative subset of the population. This was an argument presented in the ratification debates.