Monica E. Geller (born 1969/1970) is a fictional character, one of the six main characters who appears on the American sitcom Friends (1994–2004). Created by David Crane and Marta Kauffman, and portrayed by actress Courteney Cox, Monica appears in all of the show's 236 episodes, from its premiere in 1994, to its finale in 2004. A chef known for her cleanliness, competitiveness and obsessive-compulsive nature, Monica is the younger sister of Ross Geller and best friend of Rachel Green, the latter of whom she invites to live with her after Rachel forsakes her own wedding. The two characters spend several years living together as roommates until Monica begins a romantic relationship with long-time neighbor and friend Chandler Bing, whom she marries. Unable to conceive children on their own, Chandler and Monica eventually adopt twins Erica and Jack and move out of their apartment into a larger house in the suburbs.

The creators' first choice for the role of Monica was comedienne Janeane Garofalo. Cox had originally been offered the role of Rachel but declined in favor of playing the character's best friend Monica because she was drawn to her strong personality. Meanwhile, the role of Rachel went to actress Jennifer Aniston. Before Friends aired, Monica's characterization was greatly debated among writers in regards to the character sleeping with someone on their first date during the pilot. Kauffman in particular greatly defended Monica, arguing with NBC executive Don Ohlmeyer over whether or not this would make the character too promiscuous. Ultimately, the episode aired unchanged after the studio surveyed the audience, the results of which returned in favor of Monica's existing storyline. The character's struggles with childhood obesity, challenges with romantic relationships and complicated relationship with her mother would eventually become popular staples of the show.

Several months before Friends premiered, NBC had conducted a research report, the results of which determined that Monica was the only character to have been remotely well received by test audiences. When Friends first aired, critics initially perceived Monica – who was immediately established as the show's "mother hen" – and Cox as the series' main character and star, respectively. Critics have been largely receptive towards both Cox and her character; the Los Angeles Times holds Cox's acting responsible for disproving the stigma that attractive women are incapable of delivering comedic performances. Revered as a television icon, Monica famously addressed several topics that were rarely discussed in prime time television at the time, including safe sex, casual sex, and age disparity in relationships. Despite garnering positive reviews for her performance, Cox is the only main cast member never to have been nominated for an Emmy Award during Friends' ten-year run.