It's Always Sunny in Philadelphia is an American television black comedy sitcom that premiered on FX on August 4, 2005. It moved to FXX beginning with the ninth season and has remained there since. It was created by Rob McElhenney who developed it with Glenn Howerton. It is executive produced and primarily written by McElhenney, Howerton, and Charlie Day, all of whom star alongside Kaitlin Olson and Danny DeVito. The series follows the exploits of "The Gang", a group of debauched, self-centered friends who run the Irish bar Paddy's Pub in South Philadelphia.

Who is the writer of it's always sunny in philadelphia? OK. Let's think. My stream of consciousness:
To answer this question, we should know that: It is executive produced and primarily written by McElhenney, Howerton, and Charlie Day, all of whom star alongside Kaitlin Olson and Danny DeVito. The answer is McElhenney , Howerton , and Charlie Day.