In J. R. R. Tolkien's legendarium, the history of Arda, also called the history of Middle-earth, began when the Ainur entered Arda, following the creation events in the Ainulindalë and long ages of labour throughout Eä, the fictional universe. Time from that point was measured using Valian Years, though the subsequent history of Arda was divided into three time periods using different years, known as the Years of the Lamps, the Years of the Trees, and the Years of the Sun. A separate, overlapping chronology divides the history into 'Ages of the Children of Ilúvatar'. The first such Age began with the Awakening of the Elves during the Years of the Trees and continued for the first six centuries of the Years of the Sun. All the subsequent Ages took place during the Years of the Sun. Most Middle-earth stories take place in the first three Ages of the Children of Ilúvatar.

Major themes of the history are the divine creation of the world, followed by the splintering of the created light as different wills come into conflict. Scholars have noted the biblical echoes of God, Satan, and the fall of man here, rooted in Tolkien's own Christian faith. Arda is, as critics have noted, "our own green and solid Earth at some quite remote epoch in the past." As such, it has not only an immediate story but a history, and the whole thing is an "imagined prehistory" of the Earth as it is now.

The supreme deity of Tolkien's universe is Eru Ilúvatar. Ilúvatar created spirits named the Ainur from his thoughts, and some were considered brothers or sisters. Ilúvatar made divine music with them. Melkor, then the most powerful of the Ainur, broke the harmony of the music, until Ilúvatar began first a second theme, and then a third theme, which the Ainur could not comprehend since they were not the source of it. The essence of their song symbolized the history of the whole universe and the Children of Ilúvatar that were to dwell in it – Men and Elves.

Then Ilúvatar created Eä, which means "to be," the universe itself, and formed within it Arda, the Earth, "globed within the void": the world together with the three airs is set apart from Avakúma, the "void" without. The first 15 of the Ainur that descended to Arda, and the most powerful ones, were called Valar; the lesser Ainur were called Maiar.

When the Valar entered Arda, it was still lifeless and had no distinct geographical features. The initial shape of Arda, chosen by the Valar, was much more symmetrical, including the central continent of Middle-earth. Middle-earth was also originally much larger, and was lit by the misty light that veiled the barren ground. The Valar concentrated this light in two large lamps, called Illuin and Ormal. The Vala Aulë forged two great pillar-like mountains, Helcar in the north and Ringil in the south. Illuin was set upon Helcar and Ormal upon Ringil. In the middle, where the light of the lamps mingled, the Valar dwelt at the island of Almaren upon the Great Lake.

This period, known as the Spring of Arda, was a time when the Valar had ordered the World as they wished and rested upon Almaren, and Melkor lurked beyond the Walls of Night. During this time animals first appeared, and forests started to grow. The Spring of Arda was interrupted when Melkor returned to Arda, creating his fortress of Utumno (Udûn) beneath the Iron Mountains in the far north. The period ended when Melkor assaulted and destroyed the Lamps of the Valar. Arda was again darkened, and the fall of the great Lamps spoiled the symmetry of Arda's surface. New continents were created: Aman in the West, Middle-earth proper in the middle, the uninhabited lands (later called the Land of the Sun) in the East. At the site of the northern lamp was later the inland Sea of Helcar, of which Cuiviénen was a bay. At the site of the southern lamp was later the Sea of Ringil. After the destruction of the Two Lamps the Years of the Lamps ended and the Years of the Trees began. A Valian Year was considerably longer than a solar year.

After the destruction of the Two Lamps and the kingdom of Almaren, the Valar abandoned Middle-earth, moving to the continent of Aman. There they built their Second Kingdom, Valinor. Yavanna made the Two Trees, named Telperion (the silver tree) and Laurelin (the golden tree) in the land of Valinor. The Trees illuminated Valinor, leaving Middle-earth in darkness. The Years of the Trees were contemporary with Middle-earth's Sleep of Yavanna (recalled by Treebeard as the Great Darkness).

The Years of the Trees were divided into two epochs. The first ten ages, the Days of Bliss, saw peace and prosperity in Valinor. The Eagles, the Ents, and the Dwarves were conceived by Manwë, Yavanna, and Aulë respectively, but placed into slumber until the awakening of the Elves. The next ten ages, called the Noontide of the Blessed Realm, saw Varda kindling the stars above Middle-earth. This was the first time after the Spring of Arda that Middle-earth was illuminated. The first Elves awoke in Cuiviénen in the middle of Middle-earth, marking the start of the First Age of the Children of Ilúvatar, and were soon approached by the Enemy Melkor who hoped to enslave them. Learning of this, the Valar and the Maiar came into Middle-earth and, in the War of the Powers (also called the Battle of the Powers), defeated Melkor and brought him captive to Valinor. This began the period of the Peace of Arda.