Alison died in season 3 of "Teen Wolf", not because the producers wanted so, but because she decided to.
Crystal Reed was 29 years and she was doing the paper of a teenage girl, there are rumors that she didn't felt comfortable kissing the character Scott because the actor was younger (23 years old) and she felt that was to old to the serie!
Did you know that was actually Crystal who decided to say "I Love You" in the end?
BUT... Now she's back?
"Beloved 'Teen WolfÃ¢ÂÂ star Crystal Reed is making her way back to the show for the first time since Allison ArgentÃ¢ÂÂs demise in season three. Is Allison coming back from the dead? HereÃ¢ÂÂs the dealÃ¢ÂÂ¦"
OMG! It's not what everybody is thinking, YES Crystal Reed is back, but is Alison back? NO, Crystal is just going to appear on 23 episode, titled "The Maid of GÃÂ©vaudan" and she is not even playing Alison, but one of the ancestors of the Argent family.