Middlesex was a county in southeast England. Its area was almost entirely within the wider urbanised area of London and mostly within the ceremonial county of Greater London, with small sections in neighbouring ceremonial counties. The county's boundaries largely followed three rivers: the Thames in the south, the Lea to the east and the Colne to the west. A line of hills formed the northern boundary with Hertfordshire.

The historic counties of England are areas that were established for administration by the Normans, in many cases based on earlier kingdoms and shires created by the Angles, Saxons, Jutes, Celts and others. They are alternatively known as ancient counties, traditional counties, former counties or simply as counties. In the centuries that followed their establishment, as well as their administrative function, the counties also helped define local culture and identity. This role continued even after the counties ceased to be used for administration after the creation of administrative counties in 1889, which were themselves amended by further local government reforms in the years following.