Tolkien is unquestionably the most influential figure in high fantasy, and I think it is not unreasonable to say that he and C.S. Lewis were the originators of the subgenre. The Chronicles of Narnia and The Lord of the Rings were both published during the mid-1950s. Earlier in their respective literary careers, Hobbit was published in 1937 and Perelandra in 1938, the former was not yet the fully-developed high fantasy of Lord of the Rings and the latter would be better described as science fiction. Tolkien and Lewis were good friends, of course, and often discussed their writings with each other.

The roots of Tolkien's works lie primarily in medieval writings, especially Anglo-Saxon poetry and Norse epics like Beowulf and the Volsung Saga. Influences from farther afield, including the Finnish national epic the Kalevala, show up from time to time in his writings as well. Tolkien was, of course, a professor of Anglo-Saxon (and later of English language & literature) at Oxford, and was interested in poetry and languages since at least his late teenage years. In addition to being one of the all-time best selling authors in human history, he was an accomplished linguist, literary critic, and translator. The Book of Jonah in the Jerusalem Bible was translated by Tolkien, he made a complete translation of Beowulf as well as a highly influential lecture on it entitled Beowulf: The Monsters and the Critics, he did etymologies of Germanic W words for the Oxford English Dictionary, he wrote a vocabulary of Middle English, and he published translations of three Middle English poems: Sir Gawain and the Green Knight, [Pearl](http://en.wikipedia.org/wiki/Pearl_(poem)), and Sir Orfeo. One particularly interesting aspect of Tolkien's writings, from a meta-literary perspective is that they are framed as a collection of writings which he discovered and translated from Elvish (Silmarillion and related works) and the Common Tongue of Middle Earth (the Red Book of Westmarch, written by Bilbo and Frodo-- the former of whom translated some Elvish source material for use in the book). Truly an irrepressible enthusiasm for philology. Also, if you read Lord of the Rings out loud to yourself, you will find many examples where Tolkien wrote sentences that pretty much could have been lines of alliterative verse, the style of Old and Middle English that many poems were written in, including Sir Gawain & the Green Knight, Pearl, and Sir Orfeo.

Tolkien believed the world suffered from disenchantment: that along with the modernization of the Industrial and Victorian eras had come a reduced sense of wonder at the world, and a diminished willingness to believe in the fantastic and the terrible. In his essay, On Fairy-Stories (seriously, read it, it's great), he explained his views on faery-stories and the importance of fantasy and mythology. He felt that they had been tamed, that the connotation of "fairy" (he often used the spelling "Faery" or "Faerie"--the spelling was important to him, as a discriminating philologist) had become domesticated and defanged, something you would meet in your garden rather than a dark forest, something adorable rather than something which should make you tremble. He felt that such stories described the world on a spiritual plane in a way that mundane stories about the real world could not.

While at secondary school in the early 1910s, Tolkien and his three closest friends (Rob Gilson, G.B. Smith, and Christopher Wiseman) formed a private club they called the Tea Club and Barrovian Society, or TCBS for short. They talked about many things, including philology and poetry, which they also wrote and read for each other. After graduating from the boarding school, they remained in contact and visited each other. All of them considered the TCBS to be the center of their intellectual lives. It was during this period and his subsequent university education at Oxford that the seeds of his world began to grow. In 1914, he wrote a poem called The Voyage of Eärendel the Evening Star (published in the Book of Lost Tales 2), which later evolved into the culminating episode of the Quenta Silmarillion. It was based on a line of Anglo-Saxon poetry by Cynewulf. Tolkien later wrote that the name Earendil struck him as one that he could write stories about.

His influence on fantasy is profound-- anything which has Orcs in it owes him for that invention, and anything with Elves or Dwarves (although both spellings exist prior to Tolkien, he is largely responsible for the thorough dominance of Dwarves and Dwarven rather than dwarfs and dwarfish) very probably owes him as well, since neither of those mythical beings had yet taken the well-defined shape that is present in Tolkien. Elves and Dwarves were ideas present in Norse/Germanic mythology, but their physical descriptions were generally quite vague or indeterminate, and "elfs" in particular could denote a wide variety of concepts of mythological beings, from frolicsome gnomes (which one can see in The Hobbit as well as some of Tolkien's earlier writing) to wicked spirits which are responsible for causing illnesses (a view represented in late medieval Britain). To Snorri Sturluson, the medieval Icelandic writer, dwarfs were dark-elves and light elves were what we might think of. In many cases both dwarfs and elves were thought of as simply beings which populated the unknown world on the periphery of human realms, hiding in mountain caves or deep forests, sometimes practically as animistic spirits, and rarely as the magnificent, ancient civilizations that Tolkien envisioned. Any time you see a representation of invariably bearded dwarves (which, let's be honest, is any time you see dwarves), you're seeing Tolkien's influence. Elves being the noble, cultured, ancient elder race is also attributable to Tolkien-- even in situations like the game Dragon Age, which another poster mentioned, where elves are a marginalized, formerly enslaved people, they were once much more powerful than humans. That game tries to buck a lot of standard high fantasy tropes (drawing on G.R.R. Martin's work in part), but the key point is that it still existed within a context where it doesn't even make sense to the player to use the word "elves" if they're not going to be a noble ancient race. Authors like Rothfuss and Martin have specifically said that they're trying to write something in high fantasy that isn't a rehash of Tolkien. I'm deeply sorry for the 20-year-rule violations, but the magnitude of Tolkien's influence is shown by the fact that until the last 20 years or so there were not many fantasy authors making a serious effort to do something wholly apart from Tolkien. Robert Jordan's Wheel of Time series had the first installment published in 1990 and was still clearly laden with Tolkien.

After graduating from Oxford in 1915, Tolkien proposed to Edith Mary Bratt, married her in early 1916, and in the summer of 1916 shipped off to serve as a signals officer in the Great War that was then raging on the mainland-- a parting which caused both of them great distress, but it was just not very socially acceptable for a young, able-bodied man to refuse to serve in the war. He wrote the poem Kortirion Among the Trees while in a training camp in Britain in 1915-- Kortirion would later become the Elvish city on the isle of Tirion lying off the coast of Valinor. Besides disease and hostile shells, one of the chief dangers of the trenches was boredom. Tolkien's imagination had all the time it desired, and though it was nearly impossible to do any serious writing in the trenches, Tolkien nonetheless jotted down many an idea, which he would often expand upon during his time back from the trenches. "You might scribble something on the back of an envelope and shove it in your back pocket, but that's all. You couldn't write.. you'd be crouching down among flies and filth." It was during this period that he first began to dream up the two languages that would become the Elvish languages Quenya (which he worked on first) and Sindarin (which came later, in 1917, as a derivative of Quenya). Earlier in his life he had played with inventing languages, but never so thoroughly (nor with such a grasp of linguistic change) as during the period immediately after WWI. His history of Arda (the world which includes Middle-Earth) began in part as an explanation of the historical migrations of the Elves that led to the linguistic relationship between Quenya and Sindarin. The other three members of the TCBS were aleady serving in WWI by the time Tolkien joined, and they stayed in correspondence with each other regularly, writing poetry to fill their time. Rob Gilson died in the first days of the Somme offensive, which weighed heavily on the surviving three. Also at the Somme, Tolkien witnessed the advent of the tank, which made a great impression upon him, serving as inspiration for the description of Balrogs and dragons overrunning the walls of the Elvish city of Gondolin. He began writing The Fall of Gondolin, the first prose story about Middle-Earth in 1917 on the back of some military sheet music. The poems he had written prior to that point tended to become incorporated into the legendarium later on, but did not begin as poems about this other world.

(continued in next comment)

Tolkien was in many ways the father of modern fantasy as we know it. He is responsible for many of the aspects of fantasy that are see as commonplace or cliche today namley, orcs, Tolkien style elves and dwarves, dark lords (Melkor/Sauron) and many more.

Beyond that however he popularized the concept of world building. Before him almost all fantasy (there are some exceptions) was very much set in our primary world. King Arthur's legend for example is mythical and full of fantasy yet set in ostensibly the real British Isles. What Tolkien did was create a secondary world (Arda) which was almost totally divorced from our primary world. He set out to create a world for his constructed languages (another thing he popularized in fantasy to some extent) and stories which was something rather unique. Nowadays worldbuilding is a key part of many fantasy authors process. Authors from George Martin to Gene Wolfe create their own secondary worlds to house their stories and I would argue it was Tolkien, as the most successful world builder of his time, that largely is responsible for this.

In additon the genre of Mythopoeia which concerns itself with the creation of artificial mythology (named for a poem by Tolkien written to C.S. Lewis about the importance of myth) was popularized by him and the Silmarillion and Lord of the Rings remain probably the most famous works in the genre.

On his influences you can see many things from Germanic myth such as names like Gandalf or the use of goblins and trolls and of course worms (dragons). You can see the influence of the old elegiac tradition in the longing for a the golden age of the world now long past and the progressive decline in magic and the strength and feats of men/elves through the ages of the world. The Rohirrim from Lord of the Rings are heavily based on Anglo-Saxon culture in their language, poems and naming. His Christian influences are also apparent and the concept of sub-creation stems from his religious beliefs. I suppose one could say he was influenced too by Greek myth. The story of the Fall of Numenor recalls that of Atlantis. The Valar too recall the Greek Olympian gods as much as the gods of Asgard from Germanic myth.

I would say that without Tolkien the modern fantasy genre would look completely different. There have been so many imitators and those that seek to subvert the tropes that he established that I think it fair to call him at least one of the fathers of modern fantasy.

I am deleting all comments in this thread that are not about Tolkien's influence on fantasy literature. Please don't post any more discussions of Balrogs, I can hardly keep up.

I'm curious, now that I think about this - does The Wizard of Oz count as "high fantasy" by your definition?

Baum's Oz books are definitely set mostly or entirely in another world.

They're also hugely influential in ways that aren't always obvious - I don't think Isaac Asimov could have written his laws of robotics without Tik-Tok, Nick Chopper and Jack Pumpkinhead providing examples.

Baum was more prolific than Tolkien. He authored 14 Oz books and several plays/musicals - that's beyond the "authorized" sequels, which may have constituted the first "canon" of fantasy literature, in the way that some Star Wars material is canon and some isn't.

Tolkien drew on Northern European history and mythology to make up magic that feels real and authentic; Baum was a member of the Theosophical Society, and encoded a lot of real magic (or, at least, Western occultism) in his books.

If you look at a map of Oz, it makes a mandala. In the first book (altered unfortunately for the movie) there's some heavy elemental symbolism around all four of the main characters - Dorothy (who arrives by air, uses water to defeat the witch), the Scarecrow (who grows in the fields and is terrified of fire), the Tin Woodman (who yearns for a heart, but can't stand water), and the Cowardly Lion (who defines himself by fear, but is always plunging into chasms or dark spaces).

The subsequent books go even deeper in some unexpected esoteric directions and philosophical puzzles. Nick Chopper, the Tin Woodman, is a ship of Theseus - all of his parts have been replaced, until there's no original part left. In one sequel, he even meets his old head and converses with it. Which one is the real Nick, the disembodied head or the perfect replacement?

In a few of the later books, the main villain is the Nome King, a greedy, bearded fellow who lives in an underground kingdom with immense mineral wealth and a few magical items. (Which might put the hoard under Erebor into an interesting perspective.)

So there are some depths there that are a little unexpected in books for children. At least one of the sequels, Rinkitink in Oz, reads more like a Grimm's Fairy Tale than Dorothy and Toto not being in Kansas any more. (The main character is a prince who has to free his parents from enslavement and restore his kingdom. There are wars and underground traps with secret doors and other familiar things.)

So those were all published about 20 years before Tolkien wrote The Hobbit - the Judy Garland musical came out two years after The Hobbit, but that was actually the third Wizard of Oz film. The first came out in 1910 and the second in 1925. I don't think any direct links can be demonstrated between Oz and Middle Earth (despite the best efforts of some dedicated fans), but it'd be highly peculiar if Tolkien hadn't heard of Baum's books and perhaps read them to his own children.

(Both Baum and Tolkien also wrote stories about Santa Claus for their kids, but that's pretty predictable, really.)

I'm not sure if the Oz books are high fantasy - lots of sorcery, not so much swords - but I think they have to be an important ancestor.

If you're curious about Baum's vast web of influences, the best source is Michael Patrick Hearn's The Annotated Wizard of Oz.

I cannot offer much but am certain that Tolkien has written about his dislike of Wagner due to his perceived warping of Germanic folklaw.

He literally set the stage for almost all fantasy. Many common tropes found in modern fantasy were directly inspired by him. just as an example, before Tolkien, dwarves and elves were thought of as little fairy like creatures, now they are standard races in the majority of modern fantasy literature.