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.

[10] The series follows a group of self-centred, heavy-drinking, narcissistic misfits, referred to as "The Gang", who run a dirty, unsuccessful Irish bar called Paddy's Pub in the neighborhood of South Philadelphia.

Each member of The Gang exhibits unethical behavior and anti-social traits such as racism, misogyny, ableism, homophobia, transphobia, antisemitism, extreme selfishness and lack of regard for others, pathological dishonesty, narcissism, physical aggression, blatant disregard for the law, excessive drinking and substance abuse, unregulated emotions, cruelty, greed, envy, manipulative and predatory tendencies, apathy toward suffering, contempt for others and a lack of remorse.

The comedy of the show emerges from these extreme character traits resulting in conflicts that lead The Gang into absurd, dark, and painfully embarrassing situations, typically ending with them getting their comeuppance, but never learning their lesson.

This allows the show to mine a variety of socio-political and economic issues for satire and dark humor while keeping the characters in a state of relative stasis conducive to the long-running sitcom format.

Episodes usually find The Gang hatching elaborate schemes and regularly conspiring, against both outsiders and one another, for personal gain, revenge, or simply schadenfreude.

They habitually inflict physical and psychological pain on anyone who crosses their path, even each other, yet always return to the status quo at Paddy's Pub because they have alienated the rest of society and have only each other's company in the end.

Some of the subjects covered for the purpose of satire on the show include religion, masculinity, classism and income inequality, sexual coercion, stalking, racism, child sex abuse, substance abuse, blackmail, blackface, cults, gun violence, abortion rights, underage drinking, torture, police brutality and economic recession.

Frank, who comes originally from a working class background, is a multi-millionaire and is frequently shown getting away with his exploitative business schemes, including shell companies for tax evasion and sweatshops in developing countries.

Sunny also frequently engages in absurd and meta humor, pulling from a variety of television, film, theatre and musical references.

Following in the tradition of shows like Seinfeld, The Office (UK) and Curb Your Enthusiasm, the main characters of Sunny exhibit no growth except to devolve and become worse people over time, as a result of only being surrounded by like-minded narcissists who all encourage the worst aspects of each others' personalities.

The show features a core cast of five characters (The Gang) and a recurring cast of colorful side characters, including the Waitress, Cricket, the McPoyles, the Ponderosas, the Lawyer and various family members like Mrs. Kelly, Mrs. Mac and Uncle Jack Kelly, who cross paths and interact with the Gang in increasingly unhinged ways as the show progresses.

[15] The decision to make their own short films was further influenced by the release of the affordable Panasonic DVX100A digital camera as well as the accessible, low-budget look of The Office (UK) and Curb Your Enthusiasm.

The home movies were shot and reshot multiple times, initially with Hornsby playing the Mac character and McElhenney behind the camera as director.

[17] The three then developed a second "episode" of their home movie series, this time focused on the humor from Mac's sense of shame around his relationship with Carmen, a transgender woman.

[19] The original concept had "the gang" being out-of-work actors with the theme song being a cha-cha version of "Hooray for Hollywood";[22] however there were too many shows at the time with a similar premise.

Prior to Kaitlin Olson joining the show, the character Sweet Dee was going to be played by Jordan Reid, who at the time was the girlfriend of McElhenney.

"'We wanna keep it on, but we don't have any money for marketing, and we need to add somebody with some panache that we can hopefully parlay into some public relations story, just so we can get people talking.

Realizing they needed to change the trajectory of the show to please the network, McElhenney, Howerton, and Day became open to adding a new cast member who was familiar to the public.

"We had a music supervisor called Ray Espinola and we said, 'Give us everything you have in a sort of Leave It to Beaver with a big band-swing kind of feel,' and the majority of the songs are from what he sent over," Charlie Day explained.

Kiessling's work ("On Your Bike" and "Blue Blood") can also be heard during various scene transitions throughout the show, along with other composers and pieces such as Werner Tautz ("Off Broadway"), Joe Brook ("Moonbeam Kiss"), and Karl Grell ("Honey Bunch").

Songs from The Jack Gray Orchestra's album Easy Listening Symph-O-Nette ("Take A Letter Miss Jones," "Golly Gee Whiz," and "Not a Care in the World") and the John Costello III release Giants of Jazz ("Birdcage," "Cotton Club" and "Quintessential") are heard in several episodes.

Emily Nussbaum of The New Yorker praised the show, calling it "not merely the best sitcom on television but one of the most arresting and ambitious current TV series, period.

"[51] Gillian Flynn of Entertainment Weekly reviewed the first season negatively, commenting, "[I]t is smug enough to think it's breaking ground, but not smart enough to know it isn't.

"[52] Brian Lowry of Variety gave the first season a positive review, saying it was "invariably clever and occasionally a laugh-out-loud riot, all while lampooning taboo topics.

"[58] In 2015, Rolling Stone rated the top 20 greatest and funniest It's Always Sunny in Philadelphia episodes, stating "for 10 seasons, the series had mined comic gold from the execrable behavior of the owners of Paddy's Pub."

The "Gang" performed the musical The Nightman Cometh in New York City, Boston, Seattle, San Francisco, Los Angeles, and Philadelphia.

[80] Occasional guest stars include Kaitlin Olson, David Hornsby, Mary Elizabeth Ellis, Michael Naughton, Lin-Manuel Miranda, Cormac Bluestone and Danny DeVito.