"Valley Girls" is the twenty-fourth episode of the second season of The CW television series Gossip Girl. The episode served as a backdoor pilot for a potential Gossip Girl spin-off series set in the 1980s, entitled Valley Girls. The episode was directed by Mark Piznarski and written by Josh Schwartz and Stephanie Savage (this was the first episode of the series since "Much 'I Do' About Nothing" to be co-written by Schwartz). It was filmed on location in New York City, New York and in Los Angeles, California. References to elements of 1980s popular culture were heavily accentuated in the episode, which the producers hoped would bridge the generation gap between the characters and audience. "Valley Girls" aired on the CW in the United States on May 11, 2009 and was viewed live by an audience of 2.31 million Americans. Although the episode received generally positive reviews, the spin-off series was not picked up.

"Valley Girls" provides insight on the mysterious past of character Lily van der Woodsen (played by Kelly Rutherford as an adult and Brittany Snow as a teenager) through a series of flashbacks to her life as a teenager in the 80s. After getting kicked out of boarding school, Lily runs away from her wealthy parents to live in the San Fernando Valley with her sister Carol Rhodes (Krysten Ritter), the black sheep of the family. In the present, Lily's daughter Serena van der Woodsen (Blake Lively) rebels against her mother and decides to stay in jail as a statement, while her classmates prepare for the Prom. The episode introduces the main cast of Valley Girls as guest actors.

To prove she is able to handle her own affairs, Serena refuses to leave jail with either Lily or CeCe (Caroline Lagerfelt) although Lily drops the charges on which she had Serena arrested. Rufus (Matthew Settle) is also angry with Lily for Serena's arrest; Dan (Penn Badgley) and Jenny (Taylor Momsen) inform Vanessa (Jessica Szohr) that Rufus returned home without proposing to Lily, and he has remained in his bedroom ever since. Lily feuds with CeCe for telling Rufus about their lovechild. From jail, Serena encourages Blair (Leighton Meester) to enjoy prom with Nate (Chace Crawford) in the way Blair had chronicled in a scrapbook as a preteen, but each of the couple's prom plans, such as the limo, hotel reservation and Blair's dress, inexplicably go awry. Nate suspects Chuck (Ed Westwick) of sabotaging the prom in an effort to win Blair back, but Chuck denies involvement. Dan convinces Serena to allow him to pay her bail and escort her to the Prom.

At the Prom, Chuck foils a plot by Penelope (Amanda Setton), Hazel (Dreama Walker), Isabel (Nicole Fiscella), and Nelly Yuki (Yin Chang) to humiliate Blair during Prom royalty elections. Chuck admits to Serena and Dan that he has secretly been altering Nate and Blair's Prom night in order to recreate the scenes from Blair's Prom scrapbook. Meanwhile, Blair feels disconcerted while dancing with Nate and ends their relationship by the end of the night. Blair explains to Serena that after completing high school with Nate, he feels like simply a high school boyfriend. The girls reminisce about growing up together through crazy times, like sisters. Lily apologizes to Rufus and CeCe. Rufus expresses concern that Lily is too unpredictable and too much like her mother. CeCe remains indifferent, but agrees to return to Lily's home.

Throughout the episode, dialogue and objects prompt Lily to recall the events of her own first arrest. During flashbacks, a seventeen-year-old Lily Rhodes (Brittany Snow), having deliberately gotten expelled from The Thacher School in Ojai, California travels to Malibu, California to meet with her father, Rick Rhodes (Andrew McCarthy), the wealthy owner of Rhodes Records. Sadly for Lily, Rick already phoned her mother CeCe, who drove to Malibu from Montecito to deal with Lily. When Rick rejects the idea of Lily living with him in Malibu, Lily decides to find her sister Carol (Krysten Ritter) rather than move in with CeCe, whom Lily detests. Carol, an aspiring actress, had rejected the Rhode's upscale life and moved a year earlier to the San Fernando Valley. While searching for Carol, Lily meets Owen Campos (Shiloh Fernandez), who takes her to a club where they find Shep, Owen's musician friend, and Carol. Carol and Shep are in the midst of a dispute with Keith van der Woodsen (Matt Barr), the rich, antagonistic director of Shep's music video in which Carol stars, and are headed to his party to confront him for raising his price and holding the video hostage. When Lily asks why Carol does not simply use their father's company, Carol insists she does not want anyone to know of their privileged background. When the antagonism escalates to a fight at the party, security arrests Owen and Lily although Carol and Shep manage to escape. From jail, Lily calls CeCe. CeCe calls her daughters irresponsible. Carol, who has come to pay Lily's bail, overhears Lily defend Carol's lifestyle. Carol takes the phone from Lily, informs CeCe that Lily will be moving in with her, and takes Lily back to the city.

"Valley Girls" doubles as both a Gossip Girl episode and the pilot episode of Valley Girls, a possible Gossip Girl prequel. The spin-off television series would chronicle the life of Lily Rhodes while attending high school and living with Carol in 1980s Los Angeles.

Discussion about a Gossip Girl spin-off began in 2008. Despite believing the project was "unlikely," Gossip Girl executives explored potential concepts including an adaptation of the Gossip Girl book series' spin-off, The It Girl. However, they felt that The It Girl's world, centered on character Jenny Humphrey's stay at boarding school, was too small and insular to sustain a television series. They were also concerned about disrupting Gossip Girl's chemistry by taking away any of the show's cast members. In December 2009, Variety magazine reported that while the "Gossip Girl spinoff [was] still in the very early stages of development", CW had begun to consider making a backdoor pilot. Such a pilot would allow the company to evaluate viewer interest in a spin-off while saving money.

On January 14, 2009, CW green-lit a backdoor pilot for an untitled spin-off series starring a young Lily van der Woodsen. The concept was based on an original idea by Gossip Girl producers Schwartz and Savage. The pilot episode, eventually named "Valley Girls", was written by Stephanie Savage and Josh Schwartz and directed by Mark Piznarski, all of whom had previously worked the same roles for the Gossip Girl pilot. "Valley Girls" was shot on location in New York City, New York and in Los Angeles, California beginning in February 2009. Flashbacks were set apart from present day scenes through a grainy, sepia tone. A scene in which Lily conceives and carries out an "elaborate plan to kiss a boy, and then lie about it" in order to violate her school's honor code and be expelled, was cut from the final episode. "Valley Girls" aired on May 11, 2009.