London has always been a magnet attracting people not just from all over England, but from foreign places as well. Over the centuries many waves of immigrants came to London from various parts of the world, usually settling in the east of the city. Huguenots and Walloons came to escape religious persecution, especially in 1572 and 1685. Jewish communities arrived from 1656 onwards. And the Irish were also drawn to London, especially from the 1840s onwards. This means that many of us have ancestral connections to this large city.
But London wasn't always the area that we now think of as London, and to help you look for people who lived there it is important to know something of its history.
Though there are signs of early human presence on the south of the river near London, the city was first established as a major settlement by the Romans at the first possible crossing point of the river. They named this city Londinium. It covered approximately the area occupied by the Square Mile (the City of London) today and was surrounded by walls, parts of which can still be seen.
When the Romans left the settlement was abandoned, and during the Anglo-Saxon period a settlement known as Ludenwic developed slightly to the west of the old Roman city, approximately where Westminster is today. London's position made it vulnerable to Viking attacks coming up the river, so it's importance declined. King Alfred re-founded London within the old Roman walls in 886 AD, but it did not become the capital of England until the 12th century.
Until 1889 London consisted only of the City of London (the Square Mile) and a small area around it. The rest of what we know think of as London were either Middlesex, Essex or Hertfordshire to the north of the Thames, or Surrey or Kent to the south.
In the 19th century there were 97 separate parishes inside the old London Walls, and a few outside, making a total of 110 parish that comprised London. There were also ten parishes in the City of Westminster, which we now think of as part of London, not to mention those in the other counties comprising Greater London.
This situation continued until the County of London was created in 1889. Instead of the old 110 parishes, there were now 196. Then the creation of Greater London in 1965 engulfed not only the counties of London and Middlesex, but also parts of Kent, Surrey, Essex and Hertfordshire.
It is very important to understand this not just from a genealogical point of view, but also an historical one. It was the very fact that Southwark on the south bank of the Thames was not in London that enabled so many theatres, like Shakespeare's Globe Theatre, to be built there since all theatres or the production of plays within the City of London were banned from 1596.
Another reason it is so important to understand the evolution of London over time is to ensure you look for records in the right place. You might think of West Ham as being definitely part of London, but until 1965 it was part of Essex. This means that the parish registers are located in the Essex Record Office, not the London Metropolitan Archives.
There are some great historical maps of London through the centuries online, which can help you to see it's growth. Why not look at MAPCO: Map and Plan Collection Online, Old Maps of London or Maps of London. And although they don't track the early history of London, Booth's poverty maps should not be forgotten.