For a girl who is dead, Alison DiLaurentis (Sasha Pieterse) certainly appears in "Pretty Little Liars" a lot. Ali was murdered when she sneaked out of a sleepover at Spencer's (Troian Bellisario) house, and ever since then, her friends have been trying to figure out who killed her and why. The big question is, is Ali really dead? Let's look at the clues that she might still be alive.
When the show began, Ali was seen only in flashbacks, but as the story progressed, she was spotted in the present day, too. She most frequently appeared after one of her friends had some kind of mishap. Hanna (Ashley Benson) saw her when she was in hospital, after being run over by "A," and Emily (Shay Mitchell) saw her after she was almost gassed to death in a locked garage. Most recently, Ali appeared to Spencer when she was asleep. This was the biggest hint that Ali might not really be dead, because even though it could have been passed off as a dream, it was far more real than all of the other times we've seen her.
There is little doubt that Ali knew she was in trouble after snooping around to catch "A," and she left some incredible clues behind for her friends to help solve her murder. However, there was only so much she could do to prepare. "A" was responsible for setting up the pretty little liars with false clues, but clearly there was someone on their side, pointing them back in the right direction. When Ali came to Spencer in her bedroom, she gave her what she needed to find the last piece of the puzzle. Spencer's logical mind could have dreamed it, but Ali was really the only one who knew what her friends were missing.
The groundwork was laid.
The "Pretty Little Liars" Halloween episode began with Ali telling a story about twin girls who got into a fight, and one stabbed the other. It wasn't specified that the stabbed child died, but it was assumed, based on her injuries. The story Ali told was probably nothing more than a scary tale for Halloween, but it did raise the question of whether or not Alison had a twin. If she did, there is a strong possibility that it was her who was murdered, and Ali is still alive, helping her friends solve her sister's murder. The groundwork has been laid to make it possible for this to be true, even though it was laid out subtly.