Financially, Pixar has had a lot of success at the box office by going back to their previous films.  They'll be going back again next summer with Monsters University, a prequel to 2001's Monsters, Inc.  The new film shows how monsters Mike Wazowski (Billy Crystal) and James P. "Sulley" Sullivan (John Goodman) weren't always the best of friends, especially in their college days.  The trailers looked like a lot of fun, and now concept art has landed online showing the characters when they were younger.  We also get a look at supporting monsters Don, Squishy, Squishy's Mom, Art, and Terri & Terry.

Hit the jump to check out the concept art.  Monsters University opens in 3D on June 21, 2013. [Update: We've been asked by Disney to take down the concept art as this wasn't an official release.]

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Here's the official synopsis for Monsters University:

Mike Wazowski and James P. Sullivan are an inseparable pair, but that wasn’t always the case. From the moment these two mismatched monsters met they couldn’t stand each other. Monsters University unlocks the door to how Mike and Sulley overcame their differences and became the best of friends.