All 12 Seasons Of The Big Bang Theory, Ranked

"The Big Bang Theory" recently hit a streaming viewership milestone, with fans peeping 1.14 billion minutes of the 12-season sitcom in one week. That comes more than seven years after the comedy wrapped its original run on CBS. Why are people still so serious about watching these particular 30-minute packages about friendship, geek love, and the evolution of a group of nerd culture kings and queens?

What is it about "The Big Bang Theory" that made it the number one series on all of network television for the majority of its last five seasons? Is it just "Friends" in Pasadena, with better tech, but less impressive hair? Is it, like "Friends," an easy-to-watch, often genuinely clever and funny story, with occasionally heartfelt moments? Is it filled with Emmy-winning performances by a cast of mostly unknowns who became massively famous as their awkward friend group evolved with unexpected romances, word-class career accomplishments, and an unrivaled line-up of celebrity guest stars?

The answer is yes to all the above, of course. But a deeper dive into what makes the show tick reveals something else, something that's always true of series that endure as "TBBT" has. There's just a certain amount of magic, of chemistry among the cast and writers and crew, who jell to create characters and story, and a special way of unfolding them. "The Big Bang Theory" has that, and here, we break down how and when that worked best, and when it didn't, across a dozen seasons.

"Blossom" star (and real-life neuroscientist) Mayim Bialik joined "The Big Bang Theory" cast in the Season 3 finale, and it was a game changer. The budding relationship between her neuroscientist Amy Farrah Fowler and theoretical physicist Sheldon Cooper (Jim Parsons) is strange and endearing, and is one of the main narrative throughlines for the rest of the series.

The "Shamy" relationship is also a clever way to illustrate the developing social maturity for both characters, and so it is a supportive group of friends and family that end this season with Amy and Sheldon getting married. Their ceremony is officiated by one of Sheldon's heroes, "Star Wars" legend Mark Hamill. Sheldon tells him he has 4,000 things for him to sign after the wedding.

Amy and Sheldon's marriage is the final signal that most of the members of their friend group are settling into their grown-up futures. They do so surrounded by family portrayed by guest stars Kathy Bates and magician Teller as Amy's parents, Jerry O'Connell as Sheldon's big brother Georgie, and the return of Laurie Metcalf and Courtney Henggeler as Sheldon's mom and his twin sister, Missy.

But the nuptials truly are the highlight of Season 12. With the end of the series right around the corner, there is as much relief that a wedding happened as there is joy and a sci-fi all-star pay-off in watching it unfold.

As final seasons go, this one is a satisfying mix of strong comedic callbacks, and some key forward motion for the Caltech friends. Leonard (Johnny Galecki) confronts his mother (Christine Baranski) about the skimpy affection and acknowledgment she's shown him his whole life, and she apologizes for her lack of nurturing and warmth. It's perfect timing for parental preparation, as Leonard and wife Penny (Kaley Cuoco) find out they're going to have a baby.

Howard Wolowitz (Simon Helberg), in spite of having gone to space and using his connections to facilitate Sheldon meeting Mark Hamill and Stephen Hawking, is the perpetual butt of Sheldon's derision. But Howie gets a spot of revenge when he dresses like Sheldon for Halloween. When Sheldon realizes the joke is not just on him, but about him, it cuts a little deeper than anyone might have thought possible. It forces some introspection just as Sheldon and Amy's super asymmetry project wins the Nobel Prize in physics.

Also happening just in time: the series finally lands its white whale of special guest stars when "Star Trek" legend William Shatner plays himself. It was a fan-friendly ending overall, to a beloved series that had begun to overstay its primetime welcome.

Sheldon is clearly the show's breakout, scene-stealing protagonist. But the rest of their crew get their moments to shine, and in Season 8, the spotlight shines on Penny and Howard. It earns its spot in this ranking for the continuing development of two characters who don't always get the respect they deserve, but who made significant, dividends-paying moves, even in the last half of the series.

For Penny, years of chasing an acting career were looking less and less likely to lead to any big break. But thanks to Howard's wife Bernadette (Melissa Rauch), Penny gets the chance at a job as a pharmaceutical salesperson. Unsurprisingly given her charm and social intelligence, Penny is a natural at sales. She experiences career and financial success for the first time in her life, and her new confidence carries over into her relationship with Leonard. After seasons of on-again, off-again status, they're together, and end the season planning an elopement in Las Vegas.

For Howard, he's gone to space, he and Bernadette are settling into marriage, and he, too, has gained more confidence, and maturity, from his professional and personal achievements. It's with this new maturity that the longtime mama's boy has to face the death of his mother, the screeching, overprotective Mrs. Wolowitz, whose portrayer, actress Carol Ann Susi, died while Season 8 was being filmed. Howard arranges to honor his mom by inviting his friends to a dinner where they eat the last food she ever made, left in the freezer.

The biggest comic highlight of the season — and it's one of the series' all-time funniest moments — is the introduction of Footprints on the Moon, a band started by Howard and Raj Koothrappali (Kunal Nayyar). Raj is too often the least celebrated member of the friend group, but here he is front and center as he and Howard perform at Stuart's (Kevin Sussman) comic book store. Their debut tune, "Thor and Dr. Jones," is a clever, genuinely catchy ode to an imagined meet-up between Thor and Indiana Jones ("Thor and Dr. Jones, one plays with lightning, the other plays with bones").

Elsewhere, the most-oft ignored female in the group, Bernadette, announces she is pregnant, as she and Howard continue setting the pace for their whole circle embracing adulthood. Carrying over from the Season 8 finale, Season 9 starts with Penny and Leonard eloping in Vegas, which they livestream for their friends. They end the season with a formal wedding for their families, which brings guest star Judd Hirsch as Leonard's father and Keith Carradine as Penny's dad. June Squibb also guests as Sheldon's Meemaw, the first time we meet the woman with so much influence on her Moonpie.

In Sheldon and Amy storyland, the writers seem to understand that five seasons of the twosome's ups and downs is growing tiresome. In Season 9 Sheldon and Amy break up, reunite, and have sex for the first time, all by midseason. 

In what is an overall theme for the season, family-centric events find working mama-to-be Bernadette juggling her career and preparing a home and a maturing but-still-manchild husband for the pending addition to their lives. The birth of baby Halley becomes a sweet group event, as the whole gang stands together outside the hospital nursery, trying to figure out which one is Halley. One baby starts to cry, and the sound echoes the distinctive voice of Howard's late mom.

Penny and Leonard's second wedding spills over into Season 10, and Leonard worries a suspected hook-up between his dad and Sheldon's mom during the festivities could lead to Sheldon becoming his stepbrother. The wedding also brings more of Penny's family to town, as we meet her anxiety-ridden mother Susan (guest star Katey Sagal) and her former drug-dealing brother Randall (Jack McBrayer). Meeting some of the group's relatives makes it clear why they have formed their own family of friends.

The season ends on the least suspenseful cliffhanger of all, as Sheldon flies across the country to Princeton (where Amy is doing a fellowship) to propose to her. The inevitable move to permanence in their relationship, along with the birth of the group's first baby, brings Season 10 to a respectable spot on the list, especially for a late-series collection of episodes.

There is a lot to love about our introduction to this ultimately lovable collection of scientist dorks. Season 1 reveals many of the quirks and continuing storylines that will become favorites.

We see that Leonard is in love at the first sight of new neighbor Penny. We learn Leonard's roommate Sheldon is a prickly, demanding nerd the first time he points out no one else can sit in the place he likes to occupy on their sofa. We learn Raj deals with his selective mutism, which prevents him from being able to talk to women while sober, by drinking alcohol. We also learn a little too much alcohol turns him into a jerk. And we learn the words to "Soft Kitty," a lullaby Sheldon's mom sang in his childhood. He teaches it to Penny so she can soothe him when he's sick.

And in what becomes a tradition of having big season finale events, Season 1 wraps with Leonard screwing up the courage to ask Penny on a date. Penny seeks Sheldon's advice about whether or not to accept, and, in one of the first classic moments of "TBBT," he explains the Schrödinger's Cat theory to help Penny decide. She chooses to believe "the cat is alive," and the soapy saga of Penny and Leonard begins.

Appropriately, this feels like a middle of the pack season, where we find almost everyone in a holding pattern in their romantic relationships. Even Bernadette, newly married to Howard, is left alone to deal with the other Mrs. Wolowitz while Howard is off on his space adventure. It's not the out of this world experience he expected, though, thanks to the constant bullying of his fellow astronauts. He's also getting grief from the homefront, with his mom calling him on the International Space Station just to complain about his plan to live with "the little Polish girl" instead of her.

Back on planet Earth, the constant fluctuations in most of the other romances had begun to weigh down the comedy during this season, but there are a couple of all-time series great stories. Comedy legend Bob Newhart debuts as Arthur Jeffries, aka Professor Proton, the children's television show host who inspired both Sheldon and Leonard to become physicists. Newhart is delightfully feisty, with his trademark deadpan delivery in the face of two overzealous fans. But he's also touched to learn his Proton past was so meaningful to so many, and it begins a bond with Sheldon.

Then there's "The Love Spell Potential," where Howard blows away his friends (even Sheldon) and viewers with a performance as the guys' Dungeons & Dragons Dungeon Master. Little did any of us expect Howard's delightful skill of flawlessly impersonating Al Pacino, Christopher Walken, and Nicolas Cage.

One of the last of the series' truly great seasons, Season 7 finds everyone edging forward, even Sheldon, who's forced to accept change not only in his life (his first kiss with Amy!) but in his friends' lives.

Penny and Leonard get engaged, though she finds out that she has unknowingly been married to ex-boyfriend Zack (Brian Thomas Smith) for several years. Professor Proton's death rattles Sheldon so much he refuses to believe he's gone, until his idol (guest Bob Newhart, who won an Emmy for the role) visits Sheldon in a dream and tells him to prize the time he has with the people he cares about. That advice earns Leonard a rare hug from his roommate.

Sheldon has another encounter with one of his pop culture idols when he invites James Earl Jones to appear at the comic convention Sheldon wants to create. Jones invites Sheldon on a night of fun that includes a carnival, a strip club, and playing ding-dong-ditch at Carrie Fisher's house. Both Jones and Fisher guest as themselves.

Melissa Rauch and Mayim Bialik are promoted to the regular cast for Season 4, cementing a classic sitcom ensemble. Guest star visits include Steve Wozniak, LeVar Burton, George Takei, Katee Sackhoff, and Neil deGrasse Tyson, and it's a packed season with some of the series' most memorable comedy moments, that puts this season high on our ranking.

The season starts with the most Wolowitz-y moment of all the Wolowitz-y moments. Howard calls his friends for help when he gets stuck in the robotic arm he's programming. "You slipped and fell into a robot hand?" Raj asks. Howard: "Yes." Raj: "Penis first?" "Yes," Howard mumbles. He's finally freed by a bemused ER nurse, but Leonard gets another call from Howard later that same night. Leonard: "What do you mean it happened again?"

This one is textbook Sheldon. Concerned about his life expectancy, Sheldon creates a virtual version of himself: The Shelbot. He even designs a mobile screen that shows his face and allows him to interact with his friends, who find Shelbot even more annoying than the real thing.

For Stuart's New Year's Eve party at the comic book store, the guys plan to dress in Justice League costumes, but they lack a suitable candidate to look the part of Superman (Leonard in three-inch heeled boots is vetoed). They ask Penny's ex Zack to fill the suit, with Penny as Wonder Woman. The group wins the trophy for best get-up at the soiree, but when they're confronted with a carjacking in action outside, the superheroes turn and slink away in the opposite direction.

And the season finale cliffhanger: A drunken Penny and Raj wake up in bed together.

A season filled with game-changing events awaits viewers in Season 5 after that Season 4 cliffhanger is cleared up. Yes, Penny and Raj did (literally) sleep together, only after trying, unsuccessfully, to have sex. That opens the door for Penny and Leonard to reunite later in the season.

Amy goes on a date with Stuart, making Sheldon so jealous he presents her with a 31-page Relationship Agreement. She finds the gesture romantic, and they officially become a couple. Bernadette and Howard become another kind of official couple as they get married on the rooftop of The Building With the Broken Elevator, before he heads off to the International Space Station.

And in back-to-back episodes near the end of the season, Leonard Nimoy does a voiceover as Sheldon's Mr. Spock action figure, urging Sheldon in a dream to open the mint condition "Star Trek" toy Penny gives him. When he does, and breaks it, Sheldon swaps his toy with the same one Penny also gave Leonard. The Spock action figure visits him again in his dreams, and makes him feel guilty for tricking his friend, In the following episode, Howard helps Sheldon meet his ultimate science inspiration, Stephen Hawking, who makes the first of seven in-person and voiceover guest appearances on "The Big Bang Theory."

Both Melissa Rauch and Maylin Bialik made guest appearances that led to series regular roles. Nearly every installment of the season is a winner in some way. To namecheck the best-of moments in the winning Season 3: Penny and Leonard begin dating for the first time. Sheldon gets drunk to get through a public speaking situation, then hilariously embarrasses himself in front of a room full of colleagues. Penny and Leonard break up, then fight over Sheldon's loyalty like divorced parents. Sheldon tends to Penny when she injures her shoulder, even driving her to the hospital and singing "Soft Kitty" to her.

And in the quintessential visual moment of the whole series, Sheldon and Leonard star in the iconic ball pit scene. Sleep-deprived and exhausted while obsessing about a physics problem, Sheldon has a meltdown. He breaks into a playland ball pit. Leonard arrives to take him home, prompting Sheldon to swim around in the balls, popping up occasionally and saying, "Bazinga!" so Leonard can't catch him.

Season 2 is rife with humor and heart, solidifying what the series will be when it's at its best, and it's summed up by two characters and two episodes.

First, "The Panty Piñata Polarization," in which Sheldon and Penny go to war. Her days of mooching meals and Wi-Fi from Sheldon's apartment come to a halt when she offends his three-strikes policy and gets banned. She retaliates by interfering with his rigid laundry routine. "Woman, you are playing with forces beyond your ken," Sheldon warns. "Yeah, well, your ken can kiss my Barbie," Penny cleverly responds.

Their tit-for-tat continues until Leonard gives Penny Sheldon's mom's phone number. One call to Mary Cooper and Penny gets a knock on her door. It's Sheldon, offering an apology. She accepts and gets the ultimate reward: a "well-played" from Sheldon and grudging respect.

Finally, there is "The Bath Item Gift Hypothesis," the best episode of "The Big Bang Theory." When Sheldon learns Penny is giving him a Christmas gift, he plans to reciprocate with a gift of equal value. But then he opens her present, a napkin both autographed and used by Leonard Nimoy, procured by waitress Penny. "I possess the DNA of Leonard Nimoy?!" a shaking Sheldon asks. He runs to his room and returns with arms full of gift baskets, giving them all to Penny. Then he shocks her with an awkward hug.

It is Sheldon at his most sincere and open hearted, and Penny at her kindest. The gift cost her nothing in cash but is priceless to him. The episode is a classic, as a comedy episode, as a holiday episode, and as a story about sealing an unlikely friendship. It is worthy of inclusion on any list of the best TV episodes of all time.