Alex Oxlade-Chamberlain has become the latest player to cross the London divide between Arsenal and Chelsea, as the England winger has left the Gunners for the reigning Premier League champions for a reported transfer fee of £35million.
While it's not one of England's traditional derbies, the rivalry between the two has been one of the Premier League's fiercest over the last decade.
Given the enmity between the two sides, it's hard to imagine any player turning out for both sides during their career, but it's actually happened fairly often - 24 players have worn both Arsenal red and white and Chelsea blue.
Ashley Cole's move from Arsenal to Chelsea in 2006 remains one of the most controversial transfers in Premier League history, while Arsenal fans have not forgiven former hero Cesc Fabregas for choosing to join Chelsea when he left Barcelona in 2014.
See eight other players to have played for both London teams in our gallery above.
Where will Oxlade-Chamberlain rank among the most hated figures in this rivalry?