A lot of actors and actresses have walked the halls of County General.
Over the course of 15 seasons, ER has seen many a famous face. That's right, George Clooney isn't the only A-lister to appear on the medical drama. But odds are you didn't realize you were watching the next generation of stars at the time. Whether it was a guest appearance or an actor who just hadn't landed their breakout role yet, these are the celebs you probably forgot were on ER.