Greg Fidelman is an American record mixer, engineer and record producer. He is a frequent collaborator of producer Rick Rubin, and has worked with many bands in various genres, including Metallica, Black Sabbath, Slipknot, High on Fire, Red Hot Chili Peppers, Bush, Audioslave, Marilyn Manson, Slayer, and System of a Down, but has also worked on albums by U2, Johnny Cash, Neil Diamond and others.