London is the EU's financial centre and it was well reported that top banks and finance firms campaigned against Brexit.
However, it's happening anyway, so how will it affect them?
Banks will lose their 'passporting rights', which lets them sell across the EU.
Now they need to send staff abroad to set up EU offices.
Britian can set its own financial regulations now, so no bonus cap or the threat of a financial transactions tax.
But, if the rules change too much, the EU could try to block access.
The city needs workers from the EU - skilled immigrants will probably be allowed in post-Brexit but firms fear that rules will be tighter and cumbersome.
Fewer staff will mean fewer high earners paying lots of taxes.
Overall London is expected to remain Europe's financial centre and big names are re-committing to London, but the city might be in for a bumpy ride.