The Evolution Of Leonard And Sheldon's Bromance

Their first meeting

1. Their first meeting

Over the last nine seasons of The Big Bang Theory, we've loved watching Sheldon and Leonard's bromance blossom, and their bond only seems to grow stronger as the years pass. 

Let's take a look back at the tumultuous—but always entertaining—friendship of these brainy pals.

Although fans didn’t witness the guys' first meeting until Season 3 (Episode 22, "The Staircase Implementation"), we learned that Leonard was introduced to Sheldon after looking for a room to rent near Caltech. And while he was warned by former roommates and neighbors of the theoretical physicist that Sheldon was crazy, Leonard proceeded through the extensive interview process and roommate agreement before eventually moving in.

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They meet Penny for the first time

2. They meet Penny for the first time

Leonard and Sheldon were perfectly content being successful scientists when fans first met the roommates in the show's pilot episode—that was until they met their sexy new neighbor, Penny, who immediately captured Leonard’s heart much to Sheldon’s confusion. 
Their first fight

3. Their first fight

Sheldon and Leonard had their first big fight in "The Cooper-Hofstadter Polarization" (Season 1, Episode 9) after Sheldon threw away an invitation to present their joint findings at a physics conference and said their work’s success is due in large part to him.

Leonard decided to present anyway, but when he opened up the floor for a Q&A, Sheldon posed a mocking question that sparked an explosive argument between the two. The bickering got so bad, Sheldon tried to blow up Leonard’s head using only his mind, but to no avail. 
Sheldon moves out...sorta

4. Sheldon moves out...sorta

Unable to keep a secret from Leonard, Sheldon opted to move out of the apartment and attempted staying with Raj and Howard, but both kicked him out.

Upon returning to the apartment, Sheldon ultimately disclosed a sensitive piece of information about Penny to Leonard, which, thankfully, only helped the love birds' relationship blossom in "The Bad Fish Paradigm" (Episode 1, Season 2). 
Leonard gets stuck in the middle of Sheldon and Penny's fight

5. Leonard gets stuck in the middle of Sheldon and Penny's fight

Leonard got stuck in the middle of an argument after Penny accidentally shot a paintball gun at Sheldon's couch.

They later battled it out in an actual paintball game where Sheldon killed Penny because of her cushion incident. Since the blonde waitress couldn't shoot, Leonard killed Sheldon, who then broke the rules, retaliated, and killed Leonard. Luckily, the trio ended up forming a truce in "The Cushion Saturation" (Episode 16, Season 2). 
They go on an expedition together

6. They go on an expedition together

Sheldon and Leonard decided to spend one summer working together in the North Pole and it turned out their bond over science sometimes trumped Leonard's passion for Penny in "The Monopolar Expedition" (Episode 23, Season 2). 
Sheldon is jealous of Leonard

7. Sheldon is jealous of Leonard

Sheldon realized he was not the center of their friend group when Leonard and the gang started hanging out at Raj's in "The Toast Derivation" (Episode 17, Season 4). In retaliation, Sheldon tried to fit in with a new group of buds, but quickly missed his old pals—namely Leonard. 
Leonard stands up for Sheldon

8. Leonard stands up for Sheldon

When someone hacked Sheldon's online video game account, Leonard enlisted the guys to go on a quest to find the culprit and defend his roommate's honor in ​"The Zarnecki Incursion" (Episode 19, Season 4). 
Sheldon turns to Leonard for relationship advice

9. Sheldon turns to Leonard for relationship advice

When it comes to matters of the heart, Sheldon continues to turn to one person: Leonard.

In one particularly touching tête-à-tête about Sheldon's current relationship status with Amy, the neuroscientist respected Leonard's opinion so much, he conferred with his roommate about possibly taking the budding relationship with Amy to the next level in "The Flaming Spittoon Acquisition" (Episode 10, Season 5). 
They rework the roommate agreement

10. They rework the roommate agreement

Tired of the roommate agreement, Leonard refused to follow its stipulations until Sheldon agreed to the new amendments.

It took a lot of tense negotiations, but the two were able to restore their friendship—and living situation—when Sheldon agreed to institute "Leonard’s Day" as a way to celebrate his pal in "The Friendship Contraction" (Episode 15, Season 5).
Leonard moves out...sorta

11. Leonard moves out...sorta

Leonard reached his breaking point after Sheldon spoiled the ending of the sixth Harry Potter book for him. Exhausted by his roommates controlling nature, Leonard thought it'd be a good idea to move out of his apartment and cohabitate with Penny instead.

Luckily, when the move didn't go exactly as planned, Leonard was able to return to his original abode in "The Spoiler Alert Segmentation" (Episode 15, Season 6). 
Sheldon feels betrayed

12. Sheldon feels betrayed

Sheldon got angry at Leonard when he snuck back into town after returning from his trip to the North Sea early to spend some time alone with Penny on "The Deception Verification" (Season 7, Episode 2). 

Despite Leonard profusely apologizing for his fib, Sheldon began to doubt everything about their friendship and refused to forgive his friend. It's only when Howard got emotional and begged the duo to stop arguing that Sheldon decided to make amends with his roommie.

Hallelujah for Howard!
They grieve together

13. They grieve together

Sheldon and Leonard were able to lean on each other for support after finding out about the passing of their beloved childhood TV scientist, Professor Proton, in "The Proton Transmogrification" (Episode 22, Season 7). 

Friend who grieve together, stay together. 
They argue over work

14. They argue over work

Friction ensued when a magazine failed to mention Leonard in an article about the theory he co-wrote with Sheldon in "The Leftover Thermalization" (Episode 18, Season 8). However, Leonard's anger softened when a second article about the same theory made sure to mention his contribution. 
Leonard tries to move out...again

15. Leonard tries to move out...again

After getting married to Penny, Leonard hoped he could finally move in with her. But, when Sheldon learned of an impending change in his living arrangements, he decided to revert back to 2003, a simpler time before he met Leonard.

Eventually, the trio came to an agreement and Leonard decided to call off the movers, at least for the time-being, in "The 2003 Approximation" (Season 9, Episode 4). Friends forever!

Watch The Big Bang Theory on Thursdays at 8/7c on CBS and CBS All Access.