Five minutes into Girl Talk's October concert at the Starlight Ballroom in ­Philadelphia, about a hundred audience members have crowded onto the stage and are dancing as a dry-ice haze seethes around them, strobed by colored lights. Girl Talk himself--a.k.a. Gregg Gillis, a 27-year-old former biomedical engineer from ­Pittsburgh--bends over a laptop on a table at the front of the stage, a white bandanna around his head. About 15 minutes in, a shirtless young man leaps onto the table. His furious dancing loosens a cable plugged into Gillis's laptop, and the music booming through the club's P.A. speakers abruptly stops. "The people dancing on the table are not helping anyone," ­Gillis says into his microphone, as he ­fumbles with the cables taped to the floor. Booing from the audience finally shames the accidental saboteur off the stage.

Such are the perils of live performance in the age of the mashup, a fledgling art form that, like Gillis's shows, blurs the boundary between creator and consumer. A mashup is a digital recombination of musical elements extracted from different recordings--say, a vocal line from one song, a piano part from another, a drum pattern from a third. Some observers trace the form's origins to avant-garde experiments with tape loops in the 1970s, others to the "sampling" of existing recordings in 1980s rap. But the mashup is a distinctly 21st-century phenomenon, made possible by the proliferation of digital music files and the increasing quality and accessibility of software for manipulating them.

Gillis may be the most popular mashup artist in the United States. He's opened for Beck. He's performed at the rock festival Lollapalooza. His MySpace page gets more hits than that of indie-rock sensation Wilco. When he tours, he packs good-sized clubs--like the Starlight Ballroom, where more than a thousand people pressed toward the stage, dancing.

Part of Gillis's appeal is the sheer number of samples he combines in his mashups. Wikipedia lists 24 sources for "Play Your Part (Pt. 1)," the first track on his latest album, Feed the Animals. "Play Your Part" begins with a vocal by the hip-hop group UGK, paired with an instrumental track from the Spencer Davis Group's "Gimme Some Lovin'." About 42 seconds in, the rhythmic chant "Pump that shit up," from a song of the same title, succeeds the UGK lyric, and a few seconds later, "Gimme Some Lovin'" drops out. The looped fragment "Pump that shit" continues over a drum sample from a song by the Louisiana R&B singer Cupid. The vocal drops out, but the drum beat continues as the signature synthesizer arpeggios of Pete Townshend's "Let My Love Open the Door" enter. New samples succeed each other for another three and a half minutes, taking in the work of rappers Ludacris, Birdman, and T.I. and the bands Twisted Sister, Rage Against the Machine, and Temple of the Dog--among others.

A good mashup can revivify the familiar by placing it in a new context, and for the crowd in Philadelphia, that's what Girl Talk's work appeared to do. Cheers went up when recognizable instrumental lines entered the mix. Audience members sang along with sampled hip-hop lyrics and danced with an enthusiasm that the source songs probably wouldn't have inspired on their own.