Until a few years ago, most Americans thought of the ukulele—if they thought of it at all—as a fake instrument. It was just a toy, something your grandpa might've played in the living room during the family cocktail hour, or a prop for vaudeville routines. The uke had a few high-profile partisans over the years—including George Harrison, who reportedly brought them to friends' houses as gifts—but as far as the rest of the world was concerned, the ukulele stopped with "Raindrops Keep Fallin' on My Head" and Tiny Tim.

Ten or 15 years ago, things started to change. Locally, the ukulele was becoming more popular with the new wave of circus and cabaret acts (Circus Contraption, for example). Nationally, bands like the Magnetic Fields, Beirut, and the Decemberists began treating the ukulele as a serious instrument for composing songs, not just adding it as a flourish. "[The Magnetic Fields'] 69 Love Songs is primarily ukulele-based," Jason Verlinde, the publisher of Fretboard Journal said. "At the time, it was probably the best-selling ukulele record of all time."

That change hit rollercoaster speed one sunny afternoon in 2006, when a young ukulele player from Hawaii named Jake Shimabukuro sat down in Central Park and played a stunningly virtuosic version of Harrison's "While My Guitar Gently Weeps." Shimabukuro was visiting New York and playing for some segment on a local TV show, but the video found its way to YouTube and exploded—the video went viral before the term "viral video" was even coined.

Shimabukuro started playing when he was 4 and grew up in a kind of ukulele mirror universe—Hawaiians didn't think of it as a novelty. It was serious business. "I remember seeing a clip of Tiny Tim playing 'Tiptoe Through the Tulips,'" he said, "and thought, 'That's kind of, um, interesting.'"

After the video, Shimabukuro hit the big time. "I started getting tons of calls and e-mails from people all over the world," he said. "People wanting me to come play their club or venue, or other artists calling for me to play with them." Since then, Shimabukuro has traveled the world, playing with Yo-Yo Ma, Béla Fleck, Jimmy Buffett, and others. Last year, he played with Bette Midler for the Queen of England. He still seems stunned by the change in his fortunes, and the fortunes of the ukulele. "I mean, come on!" he said. "How does that happen?"

Last year, Eddie Vedder—who, like Mel Tormé and others, has used the ukulele as a go-to instrument for composing and arranging—released an entire record of ukulele songs. In 2010, the song "Hey, Soul Sister" by the band Train, a stripped-down pop song with the ukulele as its root instrument, had a long and high ride on the Billboard pop charts—not the world-music charts, Shimabukuro emphasized, but the pop charts—and was the best-selling song on iTunes that year.

If you think about it, that's huge—it's been a long time since pop music discovered a whole new instrument to add to its arsenal. Sitar, theremin, and other exotic instruments have popped in and out of style, but they were typically just used for texture, not as a foundation for songwriting.

So... what gives? Why the ukulele, and why now? Jim "Jumpin' Jim" Beloff, one of the early evangelists of the recent ukulele revival, said the instrument has a low barrier to entry but a high ceiling of possibility. "All you need to make these beautiful chord changes is these four strings," he said. "You can make a lot of chords on the ukulele with one finger, two fingers, and suddenly they're a lot simpler to do." He routinely encounters people who say they have no musical talent, and within a minute or two can teach them to play songs they know.

Then there's the internet, which not only popularized Shimabukuro, but gives easy ukulele lessons on video and allows players to form "ukulele clubs," where hundreds of people get together and play simple, popular songs in unison. The Seattle chapter has 300 members.

Beloff also has his "theory of the lapsed guitarist"—people who fooled around with guitar in high school and college but got busy with jobs and families and left the instrument in a corner of the living room. "Then you suddenly realize there's this instrument, and all the chops you had in chord changing you can use to make much more complicated chords." A novice guitar player can pick up a ukulele and suddenly feel like Django Reinhardt, easily strumming through ninths, diminished sevenths, minor sixths, and other formerly intimidating chords.

"It's like a little chord machine," Beloff said. "There are all kinds of musically sophisticated things you can wring out of those four strings." He pointed to old players like Lyle Ritz—of the elite session band the Wrecking Crew, who played for Phil Spector, the Beach Boys, the Byrds, etc.—who made extraordinary ukulele jazz records. Beloff and his wife are about to release a songbook with ukulele arrangements for works by Vivaldi, Bach, and other baroque composers.

And what about more accomplished pop and rock musicians? Why do they like to compose on the ukulele? Shimabukuro said that because of its stripped-down structure, composers don't get lost in flourishes and fancy riffs. "Everything becomes very simple," he said, "so you begin to focus on your melody... you start to choose your notes wisely."

Eddie Vedder, who has composed on the ukulele for years, found his first serious one on a surfing trip in a remote Hawaiian town. He went to the liquor store for some cases of beer and was sitting on them, waiting for a friend who'd gone to the grocery store. "I turned around and there was this ukulele hanging on the wall, right above my shoulder," he said, "just like a parrot on a pirate." He bought it and started fooling around in the sun. He'd left the case open on the sidewalk and people started throwing money into it. A new relationship was born.

"Instruments can be friends, and there's a big transition playing an instrument when it becomes your friend," he said. "You remember the day when it isn't a guest/host relationship. Most instruments take a while before they let you play them. The ukulele is different—it's a really gregarious little friend. And for its size, it's really forthright and giving. It doesn't have a Napoleon complex."

A good ukulele sounds gregarious. Vedder told one story about a night playing casually with a fellow musician. The friend was in the corner, trying to write something dark and evil-sounding on the ukulele, like it was a challenge. But he couldn't do it. They stayed up all night trying—and partying. "In the fog of the morning, he was vomiting over the balcony," Vedder said. "The uke had won!"