Every long-standing show has its defining moments. And “American Idol,” a show that changed the fortunes of both a TV network and a huge swath of the music industry, had many. These five performances, which were spread across 15 “Idol” seasons, each did something to change the course of the program. 

Jason Castro – “Hallelujah”

Jason Castro didn't wind up winning “Idol,” but this performance showed record labels and music publishers just how much its contestants could move the needle for them. Castro sang the Leonard Cohen song during Season 7, and it struck such a chord that the song instantly became a top seller again: Fans bought more than 178,000 copies of Jeff Buckley's version over the following week, and it went platinum months later. The success was so powerful that "Idol" judge Simon Cowell has basically ensured that “Hallelujah” is now performed in all his other shows, including “X Factor” and others. "You could practically see the dollar signs forming in [Simon Cowell's] eyes  before it ended," Maura Johnston, the founder of the music blog Idolator and an adjunct professor at Boston College, recalled of Castro's performance.  

Adam Lambert – “Whole Lotta Love”

While plenty of music publishers embraced the opportunity to get their older material onto the show, a number of pop music's giants held out. That included Led Zeppelin, whose members and managers spent decades rebuffing offers to use their music in contexts like “Idol.” That finally changed in Season 8, when the show's music supervisor, Robin Kaye, convinced Zeppelin that she had a contestant with the pipes to pull off "Whole Lotta Love." 

The success of the performance wound up changing Zeppelin's tune when it came to licensing their work. Though they'd licensed their songs in a handful of instances, their music can now be heard in a variety of different contexts. 

Kris Allen – “Heartless”

Allen never quite delivered on the promise he showed both during and immediately after “Idol,” but this performance wound up starting a trend that would define much of the second half of the show's run: guys with guitars dominating the show. That Allen managed to do this with “Heartless,” a Kanye West song that does not naturally lend itself to acoustic interpretation, is a testament to his talent.

This performance flared onto the Billboard Hot 100 in the week after it was released, landing at  No. 16. Those same sales were enough to land it at No. 5 on Billboard's digital songs chart. 

Candice Glover – “Lovesong”

Sure, it’s a cover of a cover; Glover’s spin on "Lovesong" took after Adele’s version of the Cure ballad, which appeared on her smash sophomore album, “21.” But her performance of “Lovesong” was so good that it broke the iron grip that men had had on the competition in recent years. Until Glover, who had been up for elimination multiple times on her way to the top, broke out and won in season 12, men had won the competition for six consecutive years.

David Cook – “Billie Jean”

Millions of fans can’t be wrong. Rolling Stone readers declared that this brooding version of the Michael Jackson classic was the best performance in the show’s history.