Pretty Little Liars is a series of young-adult novels by author Sara Shepard, beginning with 2006's inaugural entry of the same name. The series follows the lives of four girls – Spencer Hastings, Hanna Marin, Aria Montgomery, and Emily Fields – whose clique falls apart after the disappearance of their leader, Alison DiLaurentis, in the 8th grade. Later, when the girls are juniors in high school, Alison's body is found, and they begin receiving various messages from someone using the alias "A" who threatens to expose their secrets and get revenge on them for what Alison did to "A".
The novels explore several serious issues such as bullying, murder, drug addiction, underage drinking, eating disorders, homosexuality, peer-pressure, infidelity, and mental illness. Moral ambiguity and the consequences of lying are featured prominently in the series; the girls constantly create their own problems through their unwillingness to tell the truth about certain events and misdeeds they have done.