The twenty-first year of the Lollapalooza festival gets underway on Friday with performances from perhaps the most anticipated set of the evening, a reunited Black Sabbath. Tickets for the music festival sold out months ago but are in such high demand that they're currently being sold on secondary markets with the cheapest 3-day pass costing almost $400. For fans that can't make it to Chicago's Grant Park for the festivities many performances will be available for streaming on Lollapalooza's YouTube channel.

At the same time Black Sabbath takes the stage on Friday night, The Black Keys will play on the Red Bull soundstage, following a set from The Shins and indie darlings Dr. Dog earlier in the day. Sabbath's reunion has gotten the most attention, though, because of the group's stature in the music world. They're considered the founding fathers of heavy metal and reviews from their first concert back together (albeit without drummer Bill Ward, who's sitting out because of a contract dispute) in England report that the band was in fine form.

Even Ozzy Osbourne's voice is getting praise after years of criticism that the seminal frontman has lost his way in the music world. Spin Magazine has footage from Sabbath's performance at last month's Download Festival, find video footage by clicking here. They played classic rock staples  like "Iron Man" and "Paranoid" but also delved into cuts from "Black Sabbath Volume 4" by playing "Tomorrow's Dream" and "Under The Sun."

Although Sabbath will be competing with an audience with The Black Keys, probably the most popular band in modern rock after their release of "El Camino" at the end of 2011, Friday will also feature concerts from some of the most popular up-and-comers. Indie-folk harmonizers The Head and The Heart have been hailed as the next Crosby, Stills and Nash and will take the Sony stage right before M83, the mastermind of the "Drive" soundtrack.

Sharon Van Etten will also perform before metal-heads Band of Skulls take the Google Play stage at 6:00. Find Friday's full schedule at the Lollapalooza website. The rest of the weekend's acts have also been anticipated all summer as Frank Ocean, Red Hot Chili Peppers and Santigold will headline Saturday. Jack White, Justice, and Childish Gambino will all close out the festival Sunday night after a day of concert sets from The Gaslight Anthem, Florence + The Machine and Dum Dum Girls.

Lollapalooza founder and Jane's Addiction lead singer Perry Farrell seemed just as excited for the festival as concertgoers when he spoke with MTV about how the project has evolved over its twenty-one years.

"I kind of have this vision of an octopus, because the tentacles of Lollapalooza - now all of a sudden this octopus goes really out until 4 a.m. in the city of Chicago," he said. "It was great to have it here up until 10 o'clock and I know we're all out in there incredible hotels, but now we're really raising the bar when it comes to after parties. It's really stretching."

Lollapalooza will be streamed on two YouTube channels, find them by clicking here. The complete schedule is available on the festival's website, where you can create your own schedule of bands to watch.