Chandler and Monica

At the end of season 4, Chandler began an affair with his close friend Monica Geller in London. They had been friends for some time and had met when Chandler and Ross went to college together.

Chandler began dropping hints that he was interested in Monica during the first season. Chandler genuinely increased his efforts in season 3, but Monica thought he was joking and expressed little interest in a romantic relationship. At Ross and Emily's wedding in London, a somewhat intoxicated Monica and Chandler slept together. (A few years later, Monica revealed that she had actually gone to Chandler's hotel room looking to have an encounter with Joey.) They tried to keep their relationship secret from their other friends but they all eventually found out.

The two moved in together during the start of season 6 and were married at the end of season 7. Their wedding was officiated by Joey, who was ordained over the Internet. In the final episode, Monica and Chandler (who were unable to conceive) watched as their adopted twins - Erica and Jack - were born, though they had expected only a single child. Chandler has also had many problems with Monica's ex-boyfriend Richard (Tom Selleck), whom he and Joey had briefly idolized before Chandler and Monica started dating, mostly because of Richard's moustache and macho appearance.

Monica also sometimes got on Chandler's nerves because of her bossiness, neatness, and overly competitive nature. This caused Chandler to be forced into several situations, such as Monica being unable to throw a game of tennis against Chandler's boss and having to hide the fact that he is actually incredibly good at ping-pong to prevent Monica from ever entering them into doubles-tournaments.

In the alternate reality of "The One That Could Have Been" Monica is still fat and Chandler doesn't show much interest in her, but after sleeping together when Monica's current boyfriend gets called out to work during dinner, Chandler still falls in love with Monica and the two end up together anyway.

There also appears evidence that Chandler is in love with Monica even before they get together in London. In an episode of the second season Monica tries to help Chandler lose weight, even though he finds it hard to cope with Monica's hyper-active behavior, he puts up with it because Monica has just been fired from her job at 'Iridium'.