While Howard and Bernadette are recovering from their amorous bedroom activities, she asks Howard if he wanted to move in with her. He always ran home to help with his mother’s morning regiment leaving her feeling a bit cheap. Bernadette wants him all to herself. Howard does run home and gets into an argument with his mother accusing him of getting a sexual disease. She knows because she watches Dr. Phil. Howard leaves angry and ends up bunking at Leonard’s place. In the morning Sheldon wants Leonard to offer the upset Howard a hot beverage because he is upset. Then Sheldon asks why he didn't go to Raj’s, he finds out that Priya is back Leonard then leaves to visit her while Sheldon go to make Howard some tea and let him narcissistically ramble on about whatever is troubling him.
Leonard arrives and Raj forbids him from talking to Priya. They ignore him and disappear into his bedroom with Raj still forbidding it. Finally, he ends up calling them both, but he only gets their voice mails. Raj tells Leonard to call him, and then he tells Priya to have Leonard check his voice mail.
Leonard and Priya walk into the Cheesecake Factory and announce that they are going to start seeing each other. Raj begs Penny to tell Priya all of Leonard’s love making problems. Howard offers to spend two nights with Bernadette since his mother is going to Palm Springs, California. Bernadette wants a commitment from Howard and to choose between his mother and her.
Amy rushes over to Penny's place when Sheldon told her Leonard was dating Priya. She wanted to console Penny and help pick up the pieces of her broken heart that Penny said wasn't necessary. Amy still wanted to be strong for me and talk about Priya, that man-stealing bitch. Amy pointed out Penny's former boyfriend replaced her with a mate more suitable than she was. What? Priya was highly educated and came from the land that invented neat ways to have sex. Penny is a community-college dropout whose people wrote the book on tipping cows.
At Bernadette's, she answers her door and finds Howard has brought over a ton of luggage since he moved out of his mother's place. He tells her that he chooses her and that he left his mother an email. They both tell each other, "I love you," then Howard suggests they go make love in their bedroom. As they are walking to the bedroom he asks, wants to know what’s for dinner. Since she didn't have a lot in the apartment, Howard was going to send her to the store. Bernadette's expression changes from elation to what have I gotten myself into.
Leonard, Sheldon, Raj, Amy, and Priya are having pizza in apartment 4A as Penny swoops in to nibble on a few pieces. Amy invites her 'bestie' to sit next to her on the chair. Priya talks about being an international lawyer. Penny talks about her nonexistent acting career while Priya mentions she took an acting class at Cambridge University. Priya and Leonard then end up quoting a few lines from “The Taming of the Shrew” to Amy’s delight and Penny’s jealousy.
Howard and Bernadette's living arrangement doesn't seem to be working out. Howard asks for a snack, and tells her about his hypoallergenic laundry needs and that she needs to take him to the dentist. Wait a minute. Bernadette is not going to take the place of Howard's mother so Howard heads back home.
Amy again checks in on Penny. Now she describes Priya as a "smart, beautiful woman with the smoldering sexuality of a crouched Bengal tiger." Penny still says that she is fine, but now breaks into tears. Amy gave her a hug and whipped out the electrodes to record on her brain activity.
Another day, Penny went to over to Apartment 4A when she smelled Chinese food. First she is happy that Priya and Leonard weren't there, as they are spending the weekend at Catalina. It's revealed that Raj is at home forbidding the couple from dating, and Howard was (according to Sheldon), grounded by his mother for "running away" from home. Sheldon is still eating with three of them since Amy is on video conference. Amy asked if Penny was feeling better. Also Amy was able to use Penny's brainwave patterns to make a rhesus monkey cry like a disgraced televangelist. And Amy also sent Sheldon for the tea, since Penny was still upset. She didn't want any, but Sheldon told her that it wasn't optional.
- Guest starring:
- Teleplay: Bill Prady, Steven Molaro & Jim Reynolds
- Story: Chuck Lorre, Lee Aronsohn & Dave Goetsch
- The TV Critic: "It's episodes like this which keep me hoping the show can improve and get better. The attempts at characterization were small but solid. This wasn't the funniest episode you will see, nor was the Howard stuff adequately addressed but it was still encouraging." 
- The A.V. Club gave this episode a B.
- IMDb user reviews
- Title Reference: The title refers to Howard and Bernadette living together (Cohabitation).
- Chuck Lorre's vanity card.
- This episode was watched by 12.41 million people with a rating of 3.7 (adults 18-49).
- This episode aired in Canada on February 17, 2011 with 3.462 million viewers and a weekly ranking of #1.
- In the United Kingdom, this episode aired on July 21, 2011 with 1.403 million total viewers and a weekly ranking of #4.
- Episode transcript 
Sheldon wears his Green Lantern athletic jersey t-shirt and the "Are You Grid Equipped?" t-shirt from Thinker Collection.
- First appearance of Bernadette's apartment.
- Howard doesn't actually move out of his mother's house until "The Habitation Configuration" (S6E7).
- This episode is the first time Sheldon insists on bringing an upset guest a hot beverage.
- Second of five episodes to have a "Previously on The Big Bang Theory" review of past events. It summarizes "The Irish Pub Formulation" (S4E6) and is spoken by Johnny Galecki.
- Reintroduction of Priya after being absent for 9 episodes. She was first introduced in "The Irish Pub Formulation" (S4E6).
- Howard and Mrs. Wolowitz agree that all of the CSI series have their merits.
- Priya states to Raj that if you eat beef, you will have to live with cows for three months and drink their urine, but he disbelieves this portion of the Hindu code. Although rejecting her statement, a couple episodes later in "The Wildebeest Implementation" (S4E22), Raj tells Sheldon that he is taking medication that has a beta-blocker with an extract from cow's urine.
- Amy doesn't come to the The Cheesecake Factory with Priya and the four men to sit on that empty chair opposite the other end of the table where Leonard is sitting at in the Cheesecake Factory scene of this episode.
- Amy doesn't often see Bernadette and Penny work together as waitresses in the Cheesecake Factory anyway. It previously happened in "The Zazzy Substitution" (S4E3), "The Hot Troll Deviation" (S4E4) and "The Love Car Displacement" (S4E13). It also happens in "The Engagement Reaction" (S4E23).
- The first time that Howard and Bernadette are seen naked in bed together. It also happens at the end of "The Expedition Approximation" (S8E6) and it now happens in a scene of "The Sales Call Sublimation" (S9E12).
- Sheldon describes Howard as his tertiary friend, with Leonard being Howard's secondary and Raj being Howard's primary friend.
- Howard's mother wears a wig and wears fake eyebrows and he has to help.
- First time Sheldon is seen wearing a pair of jeans.
- Leonard's sister is 38 and married.
- Penny has a heightened sense of smell, especially when it comes to pizza and Chinese food, similar to Sheldon's "Vulcan hearing" (S3E1 - "The Electric Can Opener Fluctuation").
- Howard wishes a salt-related cardiac death on his mother so he and Bernadette can have the house to themselves. This comes back to bite him, with a vengeance, in Season Eight.
- This is the first episode in which Howard and Bernadette say, "I love you" to each other.
- This episode finally confirms that Penny is still not over Leonard, given that she breaks down at the thought of him being with someone else because she made a terrible mistake.
- Meanwhile, this episode has proven that Leonard has given up on Penny (at least for now).
- (Scene: Howard and Bernadette are in her bed, shortly after having sex)
- Howard: What’s so funny?
- Bernadette: Nothing. Just thinking about the noises people make during sex.
- Howard: I do sometimes get a bit carried away, don’t I?
- Bernadette: It’s cute. You sound a little like a drunken monkey. Ooh-ooh-ooh-ooh!
- Howard: You know it’s meant as a compliment.
- Bernadette: That’s how I take it.
- Howard: This is perfect. I hope this moment never ends.
- Bernadette: Me too.
- Howard: Well, gotta go.
- Bernadette: Oh! Already? Why don’t you stay over?
- Howard: Well, I’d love to, but you know my mother needs me in the morning.
- Bernadette: Please, I think the woman can manage to put a wig on by herself.
- Howard: It’s not just the wig. It’s pinning her hair up, drawing on her eyebrows. It’s a two-person job.
- Bernadette: It’s just, when you leave right after we make love, it makes me feel cheap.
- Howard: Oh, honey, I’m sorry. What can I do?
- Bernadette: Stay.
- Howard: Okay. Like, what, another five, ten minutes?
- Bernadette: Go home.
- Howard: Your call.
- Bernadette: Howard, have you ever considered us living together?
- Howard: Boy, I don’t know. You, me, Ma living under the same roof?
- Bernadette: No, I mean just you and me. You can move in here, or we can find a place.
- Howard: I’ve got a better solution.
- Bernadette: What?
- Howard: We wait for my mom’s heart to explode from all the salt she eats. Then we just stick her in the ground, flip her mattress and move into the big bedroom.
- Bernadette: Great.
- Howard: Look at us planning a future together.
- (Howard comes home from Bernadette's place. He puts his keys in the door, when he hears his mother shouting.)
- Mrs. Wolowitz: WHO'S THERE?! ARE YOU A SEX CRIMINAL?!
- Howard: Sex criminals don't have KEYS, MA!
- (Howard goes inside.)
- Mrs. Wolowitz: WHERE WERE YOU SO LATE?!
- Howard: I WAS OUT WITH BERNADETTE!
- Mrs. Wolowitz: I KNOW WHAT THAT MEANS, I WATCH DR. PHIL, AND HOPE TO GOD YOU USED A CONDOM!
- Howard: I'M NOT HAVING A CONVERSATION WITH YOU, MA!
- Mrs. Wolowitz: GOD FORBID YOU GET ONE OF THOSE "NEW FANCY" SEX DISEASES!
- Howard: NOBODY HAS A DISEASE!
- Mrs. Wolowitz: I HOPE NOT: I SHARE A TOILET WITH YOU, IS THAT WHAT YOU WANT, TO GIVE YOUR MOTHER HERPES?!
- Howard: THAT'S IT, I DON'T HAVE TO TAKE THIS..... (Howard storms out of the house and is about to leave.) AND GOOD LUCK WITH YOUR EYEBROWS IN THE MORNING!
- (Howard starts going, until he checks his pocket and he realizes he forgot something. He goes back inside to get it and his mother starts shouting again.)
- Mrs. Wolowitz: WHO'S THERE?! ARE YOU A SEX CRIMINAL?!
- Howard: Still leaving, I just forgot my CLARITIN!
- Bernadette: Wow.
- Howard: Wow, indeed.
- Bernadette: I can’t believe we’re finally living together.
- Howard: Yeah. You know what would make this moment perfect?
- Bernadette: What?
- Howard: A little snack. You got any string cheese?
- Bernadette: No. I, I might have some cheddar.
- Howard: Not as good. You can’t make strings with it. Remind me to put it on your shopping list.
- Bernadette: Okay.
- Howard: You have hypo-allergenic detergent?
- Bernadette: No.
- Howard: Better put it on the list. If you wash my underwear with regular soap, I get little red bumps on my tuchus.
- Bernadette: Wait a second. I’m doing your laundry?
- Howard: Well, honey, it’s not gonna do itself. Oh, before I forget, tomorrow morning, you’re driving me to the dentist.
- Bernadette: I have to take you?
- Howard: You don’t have to take me. You get to take me.
- Bernadette: Wait a minute. Are you telling me your mother usually takes you to the dentist?
- Howard: It’s not weird. There’s lots of kids there with their moms.
- Bernadette: I can’t believe this.
- Howard: What? It’s fun. If I have no cavities, afterwards, we go out for a treat.
- Bernadette: All right, Howard, let’s get something straight right now. I’m not going to be your mother.
- Howard: Whoa, whoa, whoa! Where did that come from?
- (It is revealed that Bernadette has kicked Howard out of her place and he has come back home to his mother's place with all of his luggage. He is putting his key in the door when his mother starts ranting again.)
- Mrs. Wolowitz: WHO'S THERE?! IS THAT A SEX CRIMINAL?!
- Howard: Nobody wants to do that TO YOU, MA!
- (Howard goes inside.)
- Mrs. Wolowitz: WHERE WERE YOU?!
- Howard: DIDN'T YOU READ MY EMAIL?!
- Mrs. Wolowitz: YOU KNOW, I CAN'T TURN ON THAT FERKAKTA COMPUTER. I LEFT YOU SOME BRISKET ON THE KITCHEN COUNTER!
- Howard: THANK YOU!
- Mrs. Wolowitz: REMEMBER TO FLOSS AFTER. WE HAVE THE DENTIST IN THE MORNING!
- Raj: It’s completely inappropriate for a single woman to entertain a man in private. If you insist on talking, you must do it on the couch! All right, you may talk in the bedroom, but I want this door to remain open! All right, just this once you may close the door. But keep in mind I’ll be right out here monitoring the situation! (Dials phone) Oh, damn it. Leonard, when you get this message; call me. (Dials again) Priya, this is your brother. When you get this, tell Leonard to check his voice mail.
- Amy: It seems like the appropriate thing to do when one’s best friend finds herself replaced by a smart, beautiful woman.
- with the smoldering sexuality of a crouched Bengal tiger.
- Penny: I already told you, I’m okay with it. I mean, if anything, I’m quite pleased (starting to blub) that Leonard has found someone (starting to cry) that makes him so happy.
- Amy: It's okay; it’s okay.
- Penny: Thank you.
- Amy: Now, let’s get these electrodes attached and see what’s going on in that pretty little noggin of yours.
- Penny: (in tears) Okay.
- Sheldon: Would you be sleeping over again this evening? Because you’re welcome to.
- Howard: That’s very nice of you, but I made other plans.
- Sheldon: Well, just keep in mind that should you ever need a slightly apathetic tertiary friend, I stand at the ready.
- Priya: Hi, Howard. Hi, Sheldon.
- Howard: Hey.
- Sheldon: Hello.
- Raj: Why are you holding hands? I forbid you to hold hands.
- Priya: Rajesh, you know Leonard and I spent the night together.
- Raj: Yeah, but you were just sleeping, because I forboded you to have sex.
- Leonard: The word is forbade.
- Raj: Are you sure? That doesn’t sound right.
- Priya: Listen, Rajesh, Leonard and I have decided to see each other again, and you don’t get to tell me who I can and can’t have a relationship with.
- Sheldon: Actually, he can. The Hindu Code of Manu is very clear in these matters. If a woman’s father is not around, the duty of controlling her base desires falls to the closest male member of her family, in this case, Raj. The code also states that if she disobeys, she will be reborn in the womb of a jackal and tormented by diseases. If true, that seems like an awfully large gamble given that the prize is Leonard.
- Raj: There it is, Priya. We’re Indian. We believe this stuff.
- Priya: I think it also says that if you eat beef, you need to live with cows for three months and drink their urine.
- Raj: Some of it makes sense, some of it’s crazy. My point is, you can’t go out with Leonard.
- Penny (arriving): Who can’t go out with Leonard?
- Raj: My sister.
- Leonard: Penny, this is Raj’s sister, Priya.
- Priya: It’s very nice to meet you.
- Penny: Oh, yeah, you, too.
- Sheldon: Priya, if you’re experiencing any tension or awkwardness, it may stem from the fact that Leonard and Penny used to, if I may quote Howard, do the dance with no pants.
- Penny: Sheldon!
- Leonard: Really?
- Howard: Hey, Bernadette, can I talk to you for a second?
- Bernadette: Sure.
- Raj: Penny, you became disenchanted with Leonard as a lover. Would you please tell my sister why?
- Howard: (He's proud and shifty) Listen, my mom’s going to Palm Springs to visit her sister. That’s two whole nights in a row I can sleep over with you all the way to morning. Unless the desert air dries out her sinuses, in which case I’ll have to schlep out there with the big humidifier.
- Bernadette: (asking Howard crossly) That’s it? That’s your big solution to all of our problems? If your mom’s nose holds up, we get two nights together?
- Howard: (he's slightly confused) Isn’t that great?
- Bernadette: (she is now very cross) No, it’s not great. You need to make a choice. Me or your mother.
- Howard: (He is even more confused) Oh, uh…
- Bernadette: (she is still very cross) Wrong answer.
- (Bernadette starts to walk away, but Howard touches her shoulder)
- Howard: (he's being assertive with her) No, wait. You didn’t let me finish.
- Bernadette: (she's still a tiny bit impatient) I’m listening.
- Howard: (mock stammering) Uh…
- Bernadette: (she groans with violent rage) Agh!
- (Bernadette finally walks away from Howard in complete fury. The Cheesecake Factory scene now fades to black).
- The Big Bang Theory - The Cohabitation Formulation (TV Episode 2011) - IMDb
- The TV Critic's Review
- The Big Bang Theory: "The Cohabitation Formulation" - TV Club - The A.V. Club
- Thursday Final Ratings: 'American Idol' Adjusted Up; 'Parks & Recreation,' 'Private Practice' Adjusted Down - Zap2it
- Top Programs – Total Canada (English) February 14 – February 20, 2011 - BBM Canada
- BARB via Wikipedia