Crossover literature transcends the conventionally recognized boundaries within the fiction market, blurring the borderline between adult literature and children’s literature. Books may cross from child to adult or adult to child audiences, or they may be explicitly published for both audiences. Crossover literature is by no means a recent phenomenon, but it received a high profile and a great deal of media attention with the unprecedented success of J. K. Rowling’s Harry Potter books in the late 1990s. It was at that time that the term “crossover” was adopted by critics, the media, and the publishing industry. New words were also coined in other languages to refer to this literature. Although the genre includes adult fiction read by young readers (adult-to-child crossover), which has a much longer historical precedent, the term is often used to refer only to children’s and young adult books that appeal to adults (child-to-adult crossover). Crossover literature is an extensive body of diverse, intergenerational works with a very long history. Borders between children’s and adult fiction have been more porous, or even non-existent, in certain cultures and time periods. Fairy tales, Middle Eastern tales, and fables have always appealed to mixed-age audiences. Children have been appropriating adult books for centuries. Classics like Robinson Crusoe and Les Trois Mousquetaires (The Three Musketeers) virtually passed into the children’s library. While almost every genre can cross age boundaries, the novel, and in particular the children’s and young adult novel, has monopolized attention. Moreover, crossover fiction is often equated with the fantasy novel, which played a key role in drawing public and critical attention to this literature. In most countries today, fantasy remains the dominant crossover genre. However, other genres, including short fiction, fairy tales, poetry, graphic novels, picturebooks, and comics commonly transcend age boundaries. Initially, many saw crossover literature as merely a marketing and mass media phenomenon, but it also received critical acclaim. Crossover fiction in the early 21st century is recognized as a distinct literary genre and marketing category. It plays a major role in the publishing industry and in contemporary culture. Crossover books are responsible for hugely successful multi-generation-spanning, cross-media franchises, such as Harry Potter, The Lord of the Rings, The Twilight Saga, and The Hunger Games. Crossover literature is part of a broader cultural trend in which books, movies, television shows, video games, and so forth are increasingly reaching across age groups.