The thirteenth season of It's Always Sunny in Philadelphia has been officially commissioned by the FXX network and will likely air in 2019 on FXX. An official start date or episode count have not been released.
In an interview published on March 8, 2017, Glenn Howerton stated that there may be an "extended hiatus" before season 13, and that he may not return to the show: "We may be taking an extended hiatus between season 12 and season 13. So I’m certainly staying open to the possibility of doing more, but there is a possibility that I will not."
In an interview with TV Guide, Kaitlin Olson confirmed that the show will take an extended hiatus to accomidate the cast's other projects: "We ended up pushing our next season a year because we were all busy with separate projects this year." So at the end of this coming shooting season of The Mick I'll step right into Sunny after that."