Not many events in the world of football provide as much sheer jubilation and immense satisfaction as a victory over your rivals.
Whether it's a fierce battle between two clubs who share the same city or indeed a hotly-contested clash between two sides whose dislike for another is purely rooted in mutual aims of success, a win over one's rivals, as well as securing coveted bragging rights, can provide the catalyst for further achievement.
Due to the inevitable fallout that can arise, most rival teams are adverse to doing business with each other. Selling a talented player to a heated rival can have huge repercussion for teams and players alike.
There have been several high-profile and hugely controversial deals completed by rival teams over the years.
Here, GMS lists five of the biggest transfer deals conducted between such rival teams. Feel free to comment on our choices and leave your own suggestions below...