The hero's journey, or monomyth, is Joseph Campbell's proposal in The Hero with a Thousand Faces (1949) that the myths of all cultures share a single underlying narrative structure. A hero leaves the ordinary world, crosses a threshold into a region of supernatural wonder, undergoes trials with the help of a mentor and companions, faces a supreme ordeal, obtains a boon, and returns transformed to benefit the community. Campbell organised it as departure, initiation and return, with numerous named stages, and drew heavily on Jung's archetypes and on the earlier pattern-work of Otto Rank and Lord Raglan.
Its cultural influence has been very large and is almost entirely outside the academy. George Lucas has repeatedly credited it in the construction of Star Wars, and through Christopher Vogler's The Writer's Journey (1992) it became a standard tool of Hollywood development and of a great deal of writing instruction. In that setting it functions as a productive template, and its usefulness there is not in dispute.
As a claim about myth it is widely rejected by specialists, and the criticisms are specific rather than merely dismissive. It selects: instances that fit are cited and those that do not are passed over, and a great many myths, those without a single protagonist, those with a female protagonist, those that end badly, and the very large body of material that is not narrative in this sense at all, do not fit. It is elastic to the point of being unfalsifiable, since the stages can be found almost anywhere once they are applied loosely enough. It abstracts stories from the cultures that tell them and from the purposes they serve there, which is the standard objection to the entire comparative-pattern tradition Campbell worked within. And it has been criticised for universalising a specifically modern, Western and individualist conception of a self who departs, is tested, and returns improved.
Feminist critics, among them Maureen Murdock, who studied with Campbell and reported his response, have argued that the pattern does not describe women's narratives and have proposed alternatives. Folklorists have generally preferred Vladimir Propp's morphology of the Russian folktale, which analyses a defined corpus and states its limits, to a scheme claiming to cover all of world myth.
The reasonable summary is that the monomyth is a poor description of the world's mythologies and an effective template for constructing modern narratives, and that its continuing authority derives from the second achievement rather than the first.