The year 1991 marked a significant change in the sound of pop music. The days of bubblegum (Debbie Gibson, Rick Astley, New Kids On The Block) and hair metal (Poison, Warrant, Motley Crue) were suddenly numbered as alternative rock and grunge culture bubbled up to the surface and heavily infiltrated the mainstream. Perhaps sensing the turning tide, veteran acts like U2 and Michael Jackson headed in new directions, musically. Meanwhile, R&B was all about new jack swing, Motownphilly and a whistle-register songbird named Mariah. Flip through our gallery to revisit 10 classic albums that celebrate their 20th anniversaries this year.