Sheldon & Amy's Relationship
Sheldon met Amy Farrah Fowler, a neurobiologist whose Internet dating profile matched the one Howard and Raj set up for Sheldon without his permission, in the third season finale after Howard blackmailed him. When Amy revealed she is there only because she has an agreement with her mother to date once a year, and informed him that all forms of physical contact, up to and including coitus, are off the table, Sheldon offered to buy her a beverage.
In the Season 4 premiere, Sheldon and Amy, who had been texting each other over the preceding four months, went on a date at the suggestion of Penny. Sheldon and Amy continued to bond throughout the fourth season, though Sheldon was always clear to point out that she was "a girl who is a friend" but she was not his "girlfriend". After Sheldon discovered feelings of jealousy when Amy went on a date with Stuart, Sheldon and Amy officially became boyfriend and girlfriend. Sheldon and Amy's relationship progressed when he cuddled Amy, comforting her after Bernadette and Penny left her out of wedding dress shopping. While watching Howard's space launch on TV, in one small step for Sheldon he grabbed hold of Amy's hand.
After Sheldon lied about his and Amy's sex life to Barry Kripke, Penny asked Sheldon whether he would ever consider having a sexual relationship with Amy. Sheldon admitted he's always been uncomfortable with the sort of physical contact that comes easily to others, but said he's working on it and that it's a possibility. For Valentine's Day, Sheldon made Amy his "In Case of Emergency" contact at the university. While the gang were playing Dungeons and Dragons, Amy got upset that they were mocking Sheldon & Amy's relationship and asked Sheldon if he thought their relationship will ever get intimate. Sheldon told Amy he doesn't consider their relationship a joke and that he would not rule intimacy out. Sheldon then played out the love potion scenario of the game with Amy, much to her pleasure.
For Valentine's Day, Amy arranged a romantic weekend trip to Napa Valley aboard a vintage train. After Sheldon spent the journey with a fellow train enthusiast, Amy confronted him about his behavior. Sheldon made Amy admit she manipulated him with the perfect trip in pursuit of romance, so Sheldon sarcastically gave her the romance she wanted. Sheldon drank wine, stared into her eyes and then kissed her. Though he intended the kiss to be short and sarcastic, Sheldon lingered and seemed like he may have enjoyed it. When Sheldon spent Anything Can Happen Thursday with Penny, she took him to see a psychic. As Sheldon was struggling with career changes, the psychic told him all his issues personally and professionally would be solved if he commits completely to Amy.
When the girls decided to throw a prom party on the roof of the apartment building, Sheldon agreed to participate in all the usual prom customs. As the night grew nearer, Sheldon was increasingly worried that Amy might be expecting something physical to happen on the night of the prom. When Sheldon went to pick up Amy to go to the party, he was overwhelmed and ran back to his bedroom. After Amy went to talk to Sheldon, she said she didn't want to miss another prom and was going to go to the party. Sheldon came out of his room to tell her he really did think she looked pretty. Amy assured Sheldon that they did not have to spend the night together just because it's prom. Amy went on to tell Sheldon there was something else she wanted to say, but she wanted him to know that he didn't have to say it back. Sheldon interrupted Amy to say he loves her too. Amy started to have a panic attack of her own.
When Amy decided to throw a Christmas party, Sheldon was unhappy at being forced to celebrate the holiday. To get revenge on Amy, Sheldon was determined to show her up with a big gift despite agreeing not to exchange gifts. Sheldon gave Amy a picture of him smiling on Santa's lap, which she loved. Although Sheldon hoped Amy would be embarrassed by not having a gift to give him, Amy had made him a box of cookies following his beloved MeeMaw's recipe. Sheldon was surprised they were both happy at Christmas. Sheldon and Amy's relationship had to cope with Sheldon's jealousy when Amy worked on a paper with Barry Kripke, and Amy's anger when Sheldon signed up for a Mars colonization project without talking to her first. When Sheldon and Amy spent an evening erecting a blanket fort in the living room, Sheldon was having so much fun that he extended the parameters of date night to allow Amy to stay over.
When Sheldon interrupted their date night kissing to ask Amy's opinion about a TV show, Amy grew angry with his lack of focus on their date. Amy took a few days away from Sheldon to consider their relationship and, after contacting Sheldon by video chat, told him she needed to take a break. Sheldon reluctantly accepted Amy's adjournment of their relationship. When Amy ended the video chat, Sheldon turned to his Gollum figure to ask what he should do with this engagement ring.
After Sheldon and Amy broke up, they both struggled to come to terms with still seeing each other even though they were no longer together. When Sheldon filmed an episode of Fun with Flags filled with veiled mentions of his split from Amy, she angrily told him to take the video down. Sheldon and Amy both had to come to terms with the idea of them seeing other people. Sheldon was upset when Barry Kripke expressed an interest in Amy, which she ultimately did not reciprocate. Sheldon's latent feelings for Amy and his upset over their break-up came to the fore when he was interviewed for a documentary about Spock. When Sheldon realized he still had feelings for Amy, he went to her apartment to talk to her but instead observed her kissing another guy. To move on from Amy, Sheldon had Howard and Raj set up an online scavenger hunt to try find him a suitable new partner. When an attractive woman solved the puzzles and showed up at his door, Sheldon sent her away despite her scientific interests because she'd missed the deadline.
For their first Thanksgiving since breaking-up, Sheldon phoned Amy to give her the Aquarium tickets since nobody else wanted to go with him. Amy asked Sheldon whether he'd be comfortable going together as friends. After they spent the day together, Amy called Sheldon to say she's ready to be his girlfriend again. Sheldon turned Amy down, telling her that getting over her was not something he excelled at. As Sheldon was gradually driven crazy by a song he had stuck in his head, he finally recognized the song, "Darlin'" by The Beach Boys, and its meaning to him. Sheldon realized the song was about his relationship with Amy, so he rushed over to Amy's apartment to get back together with her.
Back together in time for Amy's birthday, Sheldon decided to give Amy something special for her birthday - even though it meant missing the new Star Wars movie. Sheldon talked through his ideas for Amy's birthday with Penny and Bernadette, who were surprised by his decision to have coitus with Amy. When Amy heard about Sheldon's plan, she was more than up for it. After their big night together, Sheldon and Amy both agreed they enjoyed it more than they thought they would. Sheldon told Amy he is looking forward to her next birthday when they do it again.
Sheldon and Amy's newly revitalized relationship survived a visit from Sheldon's Meemaw, Constance, who was unhappy with Amy because of how she hurt Sheldon by breaking up with him. Sheldon eventually defended Amy to his Meemaw, saying they've both grown by being together. Sheldon got Meemaw to give her blessing in case Sheldon decides to hand her the engagement ring he's been keeping. For a Valentine's Day present, Sheldon added Amy's name to their Fun with Flags show, hosting a special live edition on Valentine's Day although the show ended up having very little to do with flags. For his birthday, Amy eventually convinced Sheldon to let her and the gang throw him a birthday party. After Sheldon's laptop died and Amy bought him a new one, he shared a secret with her by showing her his storage lot of old belongings he couldn't bring himself to throw away.
Leonard & Penny's Relationship
Leonard became infatuated with Penny in their very first encounter and, at the end of the first season, they finally went out on a date. Penny quickly broke things off as she feared she wasn't smart enough for Leonard, and the two characters again started dating other people.
Their relationship was rekindled in Season 3 when Leonard returned from three months in the Arctic. Penny broke up with Leonard when he told her that he loved her and she could not say the same back. Leonard and Penny's on-again/off-again relationship remained in the off state for the rest of the season. In Season 4, Leonard had a significant relationship with Raj's sister Priya, which would last until early in the fifth season.
On the 100th episode of The Big Bang Theory, Leonard spontaneously asked Penny out, to which she said yes. Following a beta test stage of their newly reformed relationship, Leonard and Penny got back together and have remained together since. Penny finally told Leonard she loved him during the sixth season episode, "The 43 Peculiarity". At the end of the sixth season, Leonard left for a fourth month expedition in the North Sea with their relationship seeming strong.
After a career setback, Penny proposed to Leonard while drunk and Leonard hesitated to answer. Leonard feared it would end their relationship, but Penny reassured him he made the right decision. After leaving her job at the Cheesecake Factory to focus on her acting career, Penny was devastated when she was fired from a part in a low-budget movie. Penny resolved to start making better decisions in her life, which she said could include her and Leonard getting married. Although Leonard initially thought Penny only brought this up because of her career problems, she told him that getting fired made her reconsider what's really important. Leonard and Penny finally agreed to get married after Leonard grabbed an engagement ring he's had in his wallet for years, got down on one knee and formally proposed to Penny.
During the eighth season, Leonard and Penny slowly adjusted to life as an engaged couple and tentatively looked ahead towards their wedding. Although Leonard and Penny considered moving in together, Sheldon's aversion to change and dependence on his best friend mean Leonard is still living in Apartment 4A. Sheldon agreed to let Leonard spend one night a week at Penny's. Leonard had to adjust to Penny's new role as the main breadwinner following her successful career change to become a pharmaceutical sales rep.
After Sheldon pushed Leonard and Penny to set a wedding date, they started to consider their wedding even though they admitted they weren't in a hurry. On the spur of the moment, Penny told Leonard that she was free tonight. When Penny told him Vegas isn't that far away, Leonard said he was in. As the couple drove to Vegas, Leonard told Penny he had something to confess: during his North Sea expedition, he kissed another woman. Penny thought Leonard was trying to sabotage their engagement by telling her about the kiss. Penny told Leonard she wasn't happy about the kiss, but she could get past it. Leonard and Penny carried on driving to Vegas.
Despite Penny's apprehension about Leonard's confession that he kissed another woman, she decides they should go through with the Vegas wedding. During the ceremony, Penny was surprised when Leonard said he had written his own vows. After Leonard's touching vows, Penny improvised a speech based on the song You've Got a Friend from Toy Story. Following their wedding ceremony, as Leonard and Penny tried to make out in their honeymoon suite, Penny told Leonard she couldn't stop picturing him kissing another woman. Things went from bad to worse after Leonard revealed that he still works with the woman he kissed. Leonard and Penny returned home and spent their wedding night in separate apartments.
In a bid to convince Penny that his kiss with Mandy was firmly in the past, Leonard spoke to Mandy to see if she would talk to Penny. When Leonard spoke to Mandy, she made him realize that he might have been trying to sabotage his relationship with Penny by telling her about the kiss. Leonard told Penny that even after all these years he still sometimes feels like he doesn't deserve her, prompting her to admit she also worries about him leaving her. They agreed to stop being so scared of losing each other and instead just be together. After Sheldon reacted poorly to Leonard and Penny's decision to live together, they decided not to make any drastic changes and instead split their time between Apartment 4A and 4B.
After Bernadette and Howard revealed that they're expecting, Penny admitted that she sometimes thinks about starting a family, though she's in no rush. During a weekend getaway to a cabin in Big Bear, Sheldon revealed to Penny that Leonard was maintaining a secret bank account, which angered Penny because she earns more money than Leonard. During their fight over Leonard's secret, Penny admitted that she isn't happy with her job but will keep it as it's the grown-up thing to do.
When Leonard's mother visited Pasadena, Penny got fed up of Beverly insulting her. Wondering why Beverly was treating her this way, she told Penny that she was hurt by her and Leonard's decision to elope to Vegas and not invite her to the wedding. To patch things up with her mother-in-law, Penny offered to have another small wedding ceremony while Beverly is still in town.
Howard & Bernadette's relationship
After Howard called in a deal he had with Leonard, Penny set Howard up with Bernadette Rostenkowski, a microbiologist who worked at the Cheesecake Factory with Penny. Though Howard and Bernadette initially appeared to have little in common, they soon bonded over their overbearing mothers and began dating. When Howard realized this could become a lasting relationship, he suddenly proposed to Bernadette. She did not accept the proposal, but the pair continued to date.
Towards the end of Season 3, Bernadette abruptly broke up with Howard. It was later revealed she dumped Howard when she caught him having cyber sex with "Glacinda the Troll" on World of Warcraft. Howard and Bernadette got back together early in Season 4.
After around six months of dating, Howard proposed to Bernadette and this time she accepted. As the engaged couple prepared for marriage, Howard began to accept Bernadette might be the breadwinner in their relationship and agreed he would stay at home if they had kids. After Wil Wheaton uploaded a video of Raj drunkenly describing some of Howard's sexual conquests, Bernadette questioned their impending nuptials. A heart-felt apology from Howard for his past behavior won Bernadette over.
As Howard's space mission approached, Bernadette decided that they should get married before he goes into space. After the couple were unable to wed at City Hall, they agreed to marry on the roof of the guys' apartment as the Google Earth satellite passed above. Sheldon, Leonard, Penny, Raj and Amy became ministers and performed the wedding.
When Howard returned from space, he and Bernadette had to adjust to life as a married couple. Bernadette eventually convinced Howard to move out of his mother's house, only to decide they should move some of Howard's stuff back so as not to break his mother's heart. For the anniversary for their first date, Howard wrote Bernadette a song which he played to her in a quarantine room, accompanied by their friends.
Following the death of his mother, Howard and Bernadette moved into Mrs. Wolowitz's old house and have worked on renovating it. Despite Howard's reluctance to change his childhood home, he eventually agreed to let Bernadette redesign the dining room. When Bernadette's father, Mike, raised the issue of starting a family, he was under the illusion that it was Howard who didn't want kids. Bernadette agreed to think about their family life when Howard promised he would help out a lot more when they had a kid, as he wants to be the kind of father who's always around.
After Bernadette revealed that she was pregnant, Howard's excitement quickly turned to dread as he started to worry about the difficulty of raising a child. Howard's realization that he will need to earn more money to take care of his family lead him to join up with Sheldon and Leonard to patent their idea of using superfluid helium in quantum vortices for guidance systems.