Mystery is a fiction genre where the nature of an event, usually a murder or other crime, remains mysterious until the end of the story. Often within a closed circle of suspects, each suspect is usually provided with a credible motive and a reasonable opportunity for committing the crime. The central character is often a detective, who eventually solves the mystery by logical deduction from facts presented to the reader. Some mystery books are non-fiction. Mystery fiction can be detective stories in which the emphasis is on the puzzle or suspense element and its logical solution such as a whodunit. Mystery fiction can be contrasted with hardboiled detective stories, which focus on action and gritty realism.

Genre fiction, also known as formula fiction or popular fiction, is a term used in the book-trade for fictional works written with the intent of fitting into a specific literary genre in order to appeal to readers and fans already familiar with that genre.