It's Always Sunny in Philadelphia is an American sitcom created by Rob McElhenney and developed with Glenn Howerton for FX. It premiered on August 4, 2005, and was moved to FXX beginning with the ninth season in 2013. It stars Charlie Day, Howerton, McElhenney, Kaitlin Olson, and Danny DeVito. The series follows the exploits of a group of narcissistic and sociopathic friends who run the Irish dive bar Paddy's Pub in South Philadelphia, Pennsylvania, but spend most of their free time drinking, scheming, arguing among themselves, and plotting elaborate cons against others, and at times each other, for personal benefit, financial gain, revenge, or simply due to boredom or inebriation.

The series has run for more seasons than any other American live-action sitcom, surpassing The Adventures of Ozzie and Harriet's 14 seasons. In December 2020, the series was renewed for a total of four additional seasons, bringing it to 18 seasons. The sixteenth season premiered on June 7, 2023. The show has received critical acclaim, with many lauding the cast performances and dark humor. It has amassed a large cult following.