Middlesex (abbreviated Middx) is a former county in southeast England. It was established in antiquity from the territory of the Middle Saxons. Proximate to the United Kingdom capital city of London, the small county became densely populated, leading to problems with its viability. It was subject to a significant loss of territory in 1889 and was abolished on 1 April 1965. The former area of Middlesex now corresponds to much of Greater London and parts of Berkshire, Hertfordshire and Surrey.

The ancient boundaries of the county were the rivers Colne, Lee and Thames, and a ridge of hills. It initially included the wealthy and politically independent City of London in the south. The City of London became a county in its own right and although separate, dominated the early administration of Middlesex.

The generally low-lying county was the second smallest by area in 1831.

Because of the proximity of London the population was unusually high for an English county — approaching three million in 1881 — which caused problems for the administration of local government and justice. In the 18th and 19th centuries the population density was especially high ...