13 Reasons Why (also stylized as TH1RTEEN R3ASONS WHY) is an American teen drama television series developed for Netflix by Brian Yorkey and based on the 2007 novel Thirteen Reasons Why by author Jay Asher. The series revolves around high school student Clay Jensen (Dylan Minnette) and the aftermath of the suicide of fellow student Hannah Baker (Katherine Langford). Before her death, she leaves behind a box of cassette tapes in which she details the reasons why she chose to kill herself as well as the people she believes are responsible for her death.

Through its various storylines, the show explores and depicts a wide range of social issues affecting modern youth. The series was produced by July Moon Productions, Kicked to the Curb Productions, That Kid Ed Productions, Anonymous Content and Paramount Television, with Yorkey and Diana Son serving as showrunners for the first season, and Yorkey for the rest of the series. Dylan Minnette and Katherine Langford star as Clay Jensen and Hannah Baker, respectively, alongside an ensemble cast. A film from Universal Pictures based on Thirteen Reasons Why began development in February 2011, with Selena Gomez set to star as Hannah, before being shelved in favor of a television series and Netflix ordering an adaptation as a limited series in October 2015, with Gomez instead serving as an executive producer.

The first season was released on Netflix on March 31, 2017. It received positive reviews from critics and audiences, who praised its themes, emotional weight, subject matter, character development and acting, particularly the performances of Minnette and Langford. For her performance, Langford received a Golden Globe Award nomination for Best Actress – Television Series Drama. However, its graphic depiction of issues such as suicide, sexual assault, bullying, and rape (along with other mature content) prompted concerns from mental health professionals. In response, Netflix added a warning card in March 2018 that plays at the start of each episode warning viewers about the themes of each season. In July 2019, Netflix edited out the suicide scene in the first-season finale.

In May 2017, Netflix renewed 13 Reasons Why for a second season due to the success of the initial 13 episodes; the second season was released on May 18, 2018, and was met with generally negative critical reviews. Coinciding with the release of the second season, Netflix released a video with the cast that cautioned viewers on some of the topics covered in the show and provided a support website with crisis numbers for people affected by depression, anxiety and other mental health issues. A third season was ordered in June 2018 and was released on August 23, 2019. In August 2019, the series was renewed for a fourth and final season, which was released on June 5, 2020. The final two seasons were also met with a negative critical response.