Heart's 'Barracuda' battles Aerosmith's 'Walk This Way' in the semifinal round of Ultimate Classic Rock's March Riff Madness.

Three weeks ago, we started with 32 of rock’s most iconic riffs. Now, that field has been whittled down to four. We’ll keep going until we theoretically arrive at the most awesome riff in all of rock history.

Heart has gone through a murderer's row of hard rock legends to get to this point, defeating Van Halen's 'Unchained,' 'Welcome to the Jungle' by Guns N' Roses and 'Kickstart My Heart' by Motley Crue. Meanwhile, Aerosmith took out Mountain's 'Mississippi Queen,' Joe Walsh's 'Rocky Mountain Way' and -- by just 13 votes -- Lynyrd Skynyrd's 'Sweet Home Alabama.'

So who moves onto the finals? It's up to you! Everyone can vote once per hour until the semifinals of March Riff Madness closes on Sunday, March 30 at 11:59PM ET. The ultimate March Riff Madness winner will be revealed on Tuesday, April 7.

Watch Heart Perform 'Barracuda'

Watch Aerosmith Perform 'Walk This Way'