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 (such as Sherlock Holmes), 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.

Mystery fiction can involve a supernatural mystery in which the solution does not have to be logical and even in which there is no crime involved. This usage was common in the pulp magazines of the 1930s and 1940s, whose titles such as Dime Mystery, Thrilling Mystery and Spicy Mystery offered what were then described as complicated to solve and weird stories: supernatural horror in the vein of Grand Guignol. That contrasted with parallel titles of the same names which contained conventional hardboiled crime fiction. The first use of "mystery" in that sense was by Dime Mystery, which started out as an ordinary crime fiction magazine but switched to "weird menace" during the later part of 1933.