High fantasy or epic fantasy is a subgenre of fantasy that is set in invented or parallel worlds. High fantasy was brought to fruition through the work of authors such as J. R. R. Tolkien and C. S. Lewis, whose major fantasy works were published in the 1950s. High fantasy has become one of the two genres most commonly associated with the general term fantasy, the other being sword and sorcery, which is typified by the works of Robert E. Howard.

High fantasy is defined as fantasy fiction set in an alternative, entirely fictional ("secondary") world, rather than the real, or "primary" world. The secondary world is usually internally consistent but its rules differ in some way(s) from those of the primary world. By contrast, low fantasy is characterized by being set in the primary, or "real" world, or a rational and familiar fictional world, with the inclusion of magical elements.

Nikki Gamble distinguishes three subtypes of high fantasy:

Where the primary world does not exist, detailed maps, geography and history of the fictional world will often be provided. The secondary world often is based on, or symbolically represents, the primary world. The Oxford of Phillip Pullman's Northern Lights is similar, a world that is "both familiar and strange". Pullman's preface to that book explains that the setting is "a universe like ours, but different in many ways".