The fifth season of the American television sitcom series It's Always Sunny in Philadelphia premiered on FX on September 17, 2009. The season contains 12 episodes and concluded airing on December 10, 2009. Beginning with this season, the series' aspect ratio was changed from to .
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The fifth season received positive reviews. On Rotten Tomatoes, it has an approval rating of 94% with an average score of 8 out of 10 based on 17 reviews. The website's critical consensus reads, "The Gang comes to accept their rotten lot in life and It's Always Sunny settles into a morbidly pleasurable groove as a comedy of foiled dreams and dunderheaded schemes."