Chandler and Monica have known each other since they were in their late teens. They met at Thankgiving 1987, when Monica was a Senior in high school and Chandler was Ross's freshman College roommate. However their relationship got off to a rocky start when Monica developed a crush on Chandler and overheard him calling her Ross's 'fat sister'. This insult motivated her to lose weight, and the following year she attempted to get him naked and humiliate him but accidentally cut off his toe instead. (The One With All The Thanksgivings).
Following this, it's unknown how or when they became friends, but when Chandler graduated from NYU, Monica (presumably having finished culinary school at a similar time and already moved into Apartment 20) cared enough to tip him off about Apartment 19 becoming available and they became neighbors.
Flashback placing a year before the show starts, imply they were closer before Joey and Rachel became their respective roommates, as they hung out at the bar alone, played pool and it was Chandler who comforted Monica about Phoebe moving out. There are hints of attraction at this point, as he told her she was "the most beautiful women he'd ever known [in real life], "one of his favourite people" and they almost have sex, only for Chandler to break the moment with a joke. (The One With The Flashback)
Their relationship in the first four seasons portrays them as close friends. In Season 2, it is revealed they've confided secrets unknown to the rest of the gang, about Monica's underwear being hung outside and Chandler having a 'third nipple'. It is Monica who takes the lead in promising Chandler he won't die alone (The One Where Heckles Dies), coaching him through his commitment issues with Janice (The One With The Metaphorical Tunnel), comforting him over his break up with Kathy (The One With Joey's Dirty Day) and even giving him detailed sex advice to better please women. (The One With Phoebe's Uterus).
It's also hinted Chandler has deeper feelings for Monica. He suggests they be each other's 'back ups' if they're both still single at 40 (The One With The Birth) and repeatedly tries to convince her to date him, seemingly offended that he's not 'boyfriend material'. Monica takes it as a joke, though she does admit he's 'sweet and smart' and that she loves him. (The One At The Beach - The One With The Jellyfish). They are also occassionally seen sitting very close or cuddling in a chair together, despite just being platonic friends.
Their actual relationship starts when Monica becomes depressed at the rehearsal dinner for Ross and Emily's wedding: her brother is getting married for the second time while she doesn't even have a boyfriend, her mother keeps making cracks suggesting she'll never get married and some drunk guy thought she was Ross' mother ("The One With Ross's Wedding, Part 2"). She decides that maybe a night of meaningless sex will make her feel better and goes looking for Joey. He isn't there, but Chandler is, and when Chandler attempts to console her, telling her he thought she was the most beautiful woman in the room that night, that she was the most beautiful woman in most rooms, Monica throws herself at him and they end up spending the night together. The next day, initially things are quite awkward between them. When they meet up at the wedding they agree that sleeping together was a stupid thing to do, but that's not enough to keep them from agreeing to meet up again that night. They say that they will stop once they leave London, but that fails and they begin a relationship. Although they go on to fall in love and have the longest romantic relationship between two of the friends throughout the series, it's not clear at this point how much of their relationship is based on genuine feelings and how much is just about sex. However, in The One With The Truth About London, Chandler refers to the first night he and Monica spent together as the most romantic night of his life, so it seems likely that Chandler, at least, already had romantic feelings for Monica by the time their first night together was over
They try to keep their relationship secret from their other friends but they all eventually found out. Joey finds out when he realizes that the two took a trip together ("The One With The Kips"). They make him promise to not tell anyone, and he agrees. Rachel overhears a telephone conversation between the two containing sexual innuendo ("The One With All The Resolutions"), although Monica denies this when she is confronted by Rachel ("The One With Chandler's Work Laugh". Rachel decides to let the two believe that she does not know. Phoebe finds out about Monica and Chandler when she and Rachel go with Ross to check out Ugly Naked Guy's apartment and, looking through the window towards Monica's apartment, she sees the two of them having sex together. Pheobe and Rachel begin to mess with Monica and Chandler which leads to Chandler saying he loves her. Ross finds out the same way in the last scene of the same episode. ("The One Where Everybody Finds Out")
The couple face new challenges with their relationship out in the open. The other's teasing about them getting married sparks Chandler's fear of commitment, leading to a fight between them. Chandler eventually proposes to prove he's not afraid to get married, but she quickly reassures him that she doesn't want marriage or kids right away and is happy to help him through his relationship issues. (The One With The Girl Who Hits Joey) Later Monica get's worried that they aren't as all over each other as Phoebe and her new boyfriend Gary, but Chandler reassures her that getting past the early stages of the relationship is more exciting for him and what they have is especially special. (The One With Rachel's Inadvertent Kiss)
In the Season 5 finale, Monica books them a romantic trip to Vegas. However Chandler is devastated when Phoebe accidentally reveals that Monica had lunch with her ex-boyfriend Richard Burke. He eventually admits to Monica that he's jealous because he knows Richard is the love of her life and he can't compare to that. Surprised, Monica promises that he's now the love of her life and comforted they affirm they've never loved anyone else as much as they've loved each other. (The One In Vegas, Part 1)
When they all go to Vegas, they're on a winning streak at the craps table when Chandler tells Monica that, if she rolls another hard eight, they should take it as a sign and get married that night. One of the dice comes up with the four and the other rolls off the table. When they go looking for it, they see it could be either a four or a five, but they agree that it's a four and decide to get married. They go to a nearby wedding chapel and are waiting to get married when a very drunk and married Rachel and Ross emerge from the wedding chapel ("The One In Vegas, Part 2"). Seeing Rachel and Ross disturbs both Monica and Chandler, and they start to think they're moving too fast. They each want to back out on the Vegas wedding, but neither of them wants to disappoint the other, so they decide they're going to leave it up to fate. However, even though they keep getting signs telling them they should get married, they feel they aren't ready yet. Chandler suggests moving in together instead and Monica enthusiastically agrees. ("The One After Vegas").
They face some problems about moving in together, as they argue over how much of Chandler's stuff should be moved and what to do with Rachel's spare room. Monica relents and makes a lot more room for Chandler's things, including his barcalounger and dog and they agree to use the room for something they both enjoy. (The One Where Ross Hugs Rachel) The rest of the series imply they adjust well to living together and no further arguments are shown.
In a flashback as well, it is implied that Chandler has had feelings for Monica after spending a night together (specifically for Monica, in which Chandler is her "first") in "The One That Could Have Been: Part 2". This shows that they were meant to be, even in a different universe!
A year later, Chandler decides he's ready to propose to Monica and he takes her out to her favorite restaurant for a romantic evening. However, the evening doesn't go as planned; Richard shows up and is seated at the next table. When they get home, still not engaged, but first Phoebe and then Rachel asks to see Monica's hand, Chandler is upset believing his surprise has been ruined. Phoebe and Joey convince him it's not, that all he needs to do is take a couple of days convincing her that he's nowhere near ready to get married, then it will still be a surprise when he actually does propose. His plan goes off a little too well, and things get even more out of hand when Richard shows up and tells Monica he still loves her and wants to marry her. Monica is very upset about how unfair it is, saying that "fair" would have been if Richard had wanted to marry her back when she was still in love with him, or if Chandler wanted to marry her now. Meanwhile, Chandler frantically searches for her, worried that he'd gone too far in pretending he didn't want to get married and that he might have ruined everything. When he returns to their apartment that evening Joey had already explained everything to Monica, and she was there waiting for Chandler with hundreds of candles burning all over the room. Monica gets down on one knee and tries to propose, but she starts crying so much she can't finish it, so Chandler gets down on one knee as well and asks her to marry him. ("The One With The Proposal, Part 2")
In Season 7, the two planned how their wedding was supposed to be but unfortunately, her parents already spent her money for buying the beach house. ("The One With Rachel's Book") Chandler has saved exactly the amount they need for Monica's dream wedding but Chandler won't spend all of his savings on one day. But Monica wants the perfect wedding Chandler will spend all the money they have but Monica decides that she wants a future and not spend their security on the wedding. Chandler tried to bond with his future Father-In-Law during a close sauna encounter backfires ("The One With Phoebe's Cookies"). Monica's mom decided to get their wedding announced in the newspaper but Chandler was unphotogenic that he can't take a good picture with Monica ("The One With The Engagement Picture") . Joey decided that he would officiate their wedding and gets his minister certificate in the internet. Monica tells the story how they got together. Chandler finds out that Monica wanted to sleep with Joey when she knocked on the door but Chandler was the only one there. Chandler is really upset and doesn't want Joey to do the wedding anymore. Joey talks to him about how Chandler and Monica are meant for each other and nothing could take that away and Chandler is happy to let Joey do the wedding. ("The One With The Truth About London").
With four weeks left before the wedding, Chandler and Monica faced with the issue of their wedding vows. With each person dealing with their own problems in writing the vows, they turn to memories of past events to help them. In the end, the only real problem is who will be saying the vows first.("The One With The Vows").
Ross makes everybody burst into laughter by threatening Chandler physically if he ever hurts his little sister. Chandler and Monica have their rehearsal dinner and after seeing his sexy celebrity mother and drag-queen dad, Chandler suddenly freaks out thinking about becoming "Mr. and Mrs Bing." He freaks out more when he hears the new answering machine message Monica recorded, so he writes a vague apology note and runs. Ross finds it and goes looking for Chandler while Phoebe and Rachel attempt to divert Monica, who is too wedding-high to notice soon anyway. Rachel and Phoebe find a positive pregnancy test in Monica's bathroom waste basket. ("The One With Monica And Chandler's Wedding, Part 1"). Ross, Phoebe and Rachel finds out that Chandler is missing, Rachel distracts Monica while the two find Chandler... in his office, the reason he was hiding because he is panicking to become the "Bings" and he'll make Monica unhappy. Ross gets him to face things one at a time and he feels better. Later smoking in the hall, Chandler overhears the girls about Monica's pregnancy test, and instead of freaking out he gets her a present and decides a baby isn't too scary. After the ceremony, Monica denies having taken a pregnancy test, Rachel seems to know more. ("The One With Monica And Chandler's Wedding, Part 2")
In Season 8 they settle in as a married couple. Although earlier episodes imply Monica planned to try for children immediately after marriage, in their first year of marriage they focus on adjusting as newly weds, supporting Ross and Rachel with their pregnancy and comforting Joey about his new-found feelings for Rachel.
In the Season 8 finale, Monica jokingly suggests they start trying for children only for Chandler to take her seriously, admitting he's crazy about them and thinks they're ready. Thrilled, they decide to start trying for immediately. ("The One Where Rachel Has A Baby, Part 2").
At the beginning of Season 9 Chandler is forced to relocate to Tulsa for work. Initially Monica plans to go with him, but is offered her dream job in New York, and they organize it so Chandler only has to be in Tulsa for half the week, although they admit it will be hard living away from each other.
After several months of a long-distance relationship Chandler learned that he would have to spend Christmas without Monica and the Friends in Tulsa, or else be fired from his job. He comes to feel empathy for the employees underneath him who are clearly depressed because they want to be spending Christmas with their families and tells them all to go home. After a female colleague named Wendy decides to stay behind to help him, Monica calls him to wish him a merry Christmas, and when he makes the mistake of mentioning that Wendy has stayed behind to help him, Monica gets suspicious and grills him, and he ends up inadvertently making it clear that he is attracted to Wendy, but assures Monica nothing will happen between them. When Wendy then hits on him, Chandler politely rejects her, saying he's a happily married man and nothing can happen between them because what he has with his wife is great. Wendy responds by asking him why he's spending Christmas with her instead of with his wife if what he has with his wife is so great. Realizing Wendy is right, Chandler decides he's had enough and quits his job before returning home to spend Christmas with Monica and the rest of the Friends, much to their delight. ("The One With Christmas In Tulsa")
Monica supports Chandler while he's unemployed and encourages him not to go back to his old job but find something he's really passionate about. She eventually helps him find an internship and subsequent job in Advertising. (The One With The Lottery).
After a year of trying to conceive, Monica and Chandler are still unsuccessful. They go get fertility tests and find out that due to Chandler's sperm having low motility and Monica's inhospitable environment, that they will most likely not conceive children on their own. Saddened by this, Monica and Chandler resolve to find another way to have kids. ("The One With The Fertility Test"). With surrogacy rejected as Monica had always dreamed of carrying her own child. Chandler finds the 'perfect guy' to donate his sperm for her, but Monica refuses saying she doesn't care if he's the most perfect guy in the world: She doesn't want to conceive any baby if it's not Chandlers. The couple decide to adopt instead. (The One With The Donor)
In The One With The Late Thanksgiving Monica and Chandler receive a call from the adoption agency saying that they were chosen by a pregnant woman in Ohio, Erica. Although there was initially some confusion during the adoption process - Erica had confused Chandler and Monica's file with a couple who were a doctor and a reverend respectively -, Chandler eventually convinced Monica to admit the truth, subsequently winning Erica over despite the lie by convincing her that they had only lied because Monica so desperately wanted a child.
In the series finale, Erica gives birth, but she didn't know that she is actually pregnant with twins, she had misinterpreted the reference to two heartbeats as referring to her heart and the baby's. Chandler and Monica subsequently name the boy Jack Bing (after Monica's father) and the girl Erica Bing (after her birth mother).
- Their sexual relationship occurred accidentally.
- The relationship started out as a secret because the producers were unsure of how the other characters would react to this unusual couple.
- Monica said the first line of the series and Chandler said the last line. ("There's nothing to tell" - "Where?")
- They have the longest romantic relationship on the show, spanning six years from the beginning of Season 5 to the end of Season 10. Their closest competitors were Ross/Rachel and Phoebe/Mike who were both together for a year (halfway through Season 2 - halfway through Season 3, and all of Season 10).