Sheldon and Leonard's friendship was a large focal point on The Big Bang Theory. They weren't just colleagues and roommates — they were best friends. As different as they were, there was a bond between them that couldn't be ignored. No matter how annoying Sheldon could be at times, Leonard had a soft spot for him and couldn't give up on him.
As the series progressed and the two friends aged, they bickered more but ultimately grew together. By not sharing the same living space, the two friends actually became closer. From season 1 to season 12, Leonard and Sheldon's friendship prevailed.
Season 1
As is the case with most sitcoms, the first season of The Big Bang Theory tested things out with its characters to see what would stick. When it came to Sheldon and Leonard’s friendship, the two started the series trying to sell their sperm for money and ended with them becoming best friends with their new neighbor, Penny.
Season 2
Fans got to know Sheldon and Leonard even more in the second season. Penny and Leonard began to casually date, which showed a new dynamic between the guys. The same thing happened when Leonard dated Dr. Stephanie Barnett in this season.
Sheldon became attached to both women and grew annoyed when it didn't work out. By the end of the season, Sheldon asked Leonard, Howard, and Raj to join him on a three-month expedition to the North Pole for work.
Season 3
At the start of season 3, Sheldon and Leonard were at each other's throats when Sheldon found out that Leonard butchered his project in the North Pole. The two eventually became civil again but Sheldon definitely lost trust in his roommate and best friend.
It's also in this season that Leonard and Penny started a serious relationship but their bickering began to affect Sheldon. It reminded him of his youth in Texas and he hated it. But just as it happened in season 2, Leonard and Penny broke up again. Leonard and Sheldon had their ups and downs but by the end of the season, Sheldon was about to meet his future wife.
Season 4
Sheldon and Leonard's relationship began to change in season 4 when Leonard began dating Raj's sister Priya. Sheldon and Priya did not get along and Leonard began rebelling against his roommate. When Leonard and Priya broke up, Sheldon was thrilled to have a sense of peace back in the apartment.
It also changed when Sheldon became closer with Amy. Amy wasn't the easiest person to get along with in season 4 and Leonard wasn't very fond of her.
Season 5
In the fifth season, Sheldon and Amy became official at around the same time Leonard and Penny gave their love another chance. In "The Friendship Contraction," their friendship hit a snag when Sheldon's emergency preparedness drill disturbed Leonard enough to break the roommate agreement. This meant that Sheldon and Leonard were no longer best friends — they were now acquaintances.
Season 6
When Sheldon needed more help at work, he hired an assistant named Alex. Alex was brilliant, beautiful, and had a crush on Leonard. Once Sheldon found out about her feelings, he was bothered by both parties, even though Leonard was with Penny at the time. Things escalated in "The Spoiler Alert Segmentation," when a fight forced Leonard to move out and move in with Penny. The argument didn't last long, of course, and all was well by the end of the season when Leonard traveled to the North Sea.
Season 7
Sheldon missed Leonard when he was away. It proved that while they bicker and pick at each other, there was a lot of love between them. In "The Proton Displacement," the two became close with a new mentor, Arthur Jeffries (aka Professor Proton). When Arthur passed away at the end of the season, Sheldon hugged Leonard out of sheer sadness in one of the most emotional scenes in the series.
Season 8
Sheldon had a hard time at the end of season 7 and went on a train ride throughout the summer to clear his mind. But at the start of season 8, Sheldon found himself needing a ride from Arizona. Leonard dropped everything to rescue him, which wasn't commended enough. Their bond was shown again in "The Septum Deviation" when Sheldon skipped work to make sure Leonard's nasal surgery went well. The two even came together for a superfluid vacuum theory that got them recognized by other professionals in their field.
Season 9
At the start of season 9, Leonard and Sheldon were at a crossroads. Leonard and Penny were now married but Sheldon was now single after Amy broke up with him. Sheldon relied heavily on Penny and Leonard for support during this time. Focusing on work, Sheldon and Leonard came together for a hypothesis that required liquid helium. The two embarked on a hilarious adventure that could have ended horribly wrong. And by the end of season 9, Sheldon and Leonard joined forces with Howard for a project for the US Air Force.
Season 10
Sheldon and Leonard are still best friends in season 10 but found themselves starting anew. Leonard and Penny got remarried in front of friends and family, and Sheldon officially moved into Penny's old apartment with Amy. If anything, living apart made their bond closer because they didn't need to spend so much time together. The season ended on a cliffhanger when Sheldon flew from Los Angeles for New Jersey to propose to Amy.
Season 11
The final two seasons of the series were huge for Sheldon, and he was lucky to have Leonard nearby for help. Living and working together for all those years really paid off for their friendship. In "The Retraction Reaction," the two friends get into an argument over the future of physics but evened out in time for Sheldon's wedding. Leonard helped Sheldon plan his dream wedding with Amy and even flew down to Texas to try and get Sheldon's brother to come.
Season 12
It was hard for fans to say goodbye to Leonard and Sheldon but they had a happy ending. With both men now married and living across the hall from each other, they continued helping each other at work. Sheldon and Amy did groundbreaking work themselves and won a Nobel Prize for their efforts. Along for the entire ride were Leonard and co. The final scene of the series proved that everything changed but their friendship was as strong as ever.