Following the breakup of Engineering an Empire, guitarists Rich Castillo and Mike DiGiulio and bassist Jacob Nycz recruited drummer Justin Barone. The band started writing under the name Consonance but had trouble finding a vocalist. Eventually bassist and producer Nick Shann replaced Nycz when he decided that he would devote his time only to playing French Horn. Shortly after, they were joined by Dylan Barone on vocals and keys. The band recorded an EP with Shann who was later replaced by Robbie Liebold.
Twenty-five years and my life is still trying to get that great big hill of hope for a destination. I realized quickly when I knew I should that the world was made up of this brotherhod of man for whatever that means
And so I cry sometimes when I'm lying in bed just to get it all out what's in my head And I, I am feeling a little peculiar. And so I wake in the morning. And I step outside. And I take a deep breath and I get real high. And I scream from the top of my lungs What's going on?
The show began as a short film idea written by Rob McElhenney and Glenn Howerton about a man telling his friend he might have cancer, while the friend is only intent on trying to borrow a cup of sugar for the "shitload of coffee" he has made. This was then developed into a pilot called It's Always Sunny on TV and was shot on a digital camcorder by Charlie Day, Glenn Howerton, and Rob McElhenney. This pilot was shopped by the actors around various studios, their pitch being simply showing the DVD of the pilot to executives. After viewing the pilot, FX Network ordered the first season. Although it is often stated publicly that Always Sunny was one of the first shows to be shot in 24p standard-definition video, using Panasonic's DVX100 MiniDV prosumer video camera, from the sixth season forward, the show was shot in 24p using high-definition video cameras.
Danny DeVito joined the cast in the first episode of the second season, playing the father of Dennis (played by Glenn Howerton) and Dee (played by Kaitlin Olson).
The first season ran for seven episodes with the finale airing September 13, 2005. According to McElhenney, word of mouth on the show was good enough for FX to renew it for a second season, which ran from June 29 to August 17, 2006. Reruns of edited first-season episodes began airing on FX's parent network, Fox, in June 2006, for a planned three-episode run—"The Gang Finds a Dead Guy", "Gun Fever" (which was renamed as "Gun Control") and "Charlie Gets Molested" were shown. The show would not be shown on broadcast television again until 2011, when FX began offering the show for syndication.
The third season ran from September 13, 2007 to November 15, 2007. On March 5, 2008, FX renewed It's Always Sunny in Philadelphia for a fourth season. On July 15, 2008, it was reported that FX had ordered 39 additional episodes of the series, consisting of the fifth and sixth seasons. All five main cast members were secured for the entire scheduled run. The fifth season ran from September 17, 2009 to December 10, 2009. On May 31, 2010, Comedy Central began airing reruns of It's Always Sunny in Philadelphia.WGN America also began broadcasting the show as part of its fall 2011 schedule.
The sixth season ran from September 16, 2010 until December 9, 2010, running twelve episodes, plus the Christmas special. The seventh season ran from September 15, 2011 until December 15, 2011, running 13 episodes. On August 6, 2011, FX announced it had picked up the show for an additional two seasons (the show's eighth and ninth) running through 2013. On March 28, 2013, FX renewed the show for a tenth season, along with the announcement that the series would move to its new sister network, FXX.