Although all Men are related to one another, there are many different groups with different cultures. The most important group in the tales of the First Age were the Edain. Although the word Edain technically refers to all Men, the Elves used it to distinguish those Men who fought with them in the First Age against Morgoth in Beleriand. The Edain were divided into three Houses.

Further east of Umbar another group of Men lived, called the Haradrim or Southrons. They were dark skinned Men and waged war on great Oliphaunts or Mûmakil. They too were hostile to Gondor, though they were subdued in 1050 of the Third Age by Hyarmendacil I. Both Umbar and the Harad were left unchecked by Gondor's waning power by the time of the War of the Ring, and presented grave threats from the south. Many Haradrim fought with Sauron's forces in Gondor in that War.

The Easterlings of the Third Age were divided in different tribes, such as the Wainriders and the Balchoth. The Wainriders were a confederation of Easterlings who were very active between 1856 and 1944 TA. They posed a serious threat to Gondor for many years, but were utterly defeated by Eärnil II in 1944.

At Oromë's urging, many of the Elves (especially the kindreds of Ingwë, Finwë, and Elwë) agreed. But others, henceforth called the Avari, declared that they preferred starlight and the wide spaces of Middle-earth. So the Elves were first sundered. During the journey to Belegaer, gradually the number of the Elves began to lessen as various groups dropped away. Some of the Teleri (kindred of Elwë) refused to cross the Misty Mountains, and settled in Anduin under the leadership of Lenwë, to be called later the Nandor. Elwë then went missing, and in dismay the rest of the Teleri remained behind, while the Noldor (kindred of Finwë) and Vanyar (kindred of Ingwë) used an island as a ship, and found at last Aman and Valinor.

At this time, there were three Hobbit-kinds, with different temperaments. The Harfoots, the most numerous, were almost identical to the Hobbits as they are described in The Lord of the Rings. The Stoors had an affinity for water, boats and swimming; the Fallohides were an adventurous people. (Both of these traits were much rarer in later days.) While situated in the Valley of the Anduin River the Hobbits lived close by the Eotheod, the ancestors of the Rohirrim, and this led to some contact between the two.

Durin's folk settled in Dunland, and in Third Age 2790 King Thrór gave to his son the Last of the Seven Rings of the Dwarves, and with Nár he traveled North to Moria. But he was killed there by the Goblins king Azog. Nár returned and enraged, Thráin, called to all the Houses of Dwarves to war, to avenge his father. Thus began the War of Dwarves and Orcs, in which the Dwarves destroyed all the Goblin strongholds in the Misty Mountains one by one, until they came to the East-gate of Moria, where the great Battle of Azanulbizar was fought. In this battle all the dwarven clans united, but the Goblins were still slowly winning, until at last help came - Náin son of Grór came with fresh Dwarven forces from the Iron Hills. The Battle ended with the victory of Dwarves, but at great cost. Náin was slain by Azog, but his son Dáin avenged him by killing Azog. Thráin wanted to enter Khazad-dûm, but the other Dwarven clans would not, and Dáin also warned him of the Durin's Bane that he felt when he came close to the Gate.

It is unclear when exactly Orcs were created, but it certainly happened before the War for Sake of the Elves in his stronghold of Utumno. If the Orcs where at this time a capable fighting force against the host of Valinor is not known. But at least some of them survived this war, probably hidden in the deep vaults of Angband and multiplied, waiting for their master.

The Orcs of Mordor fought in major battles during the War of the Ring, such as the Battle of the Pelennor Fields, but the entire Mordor force was destroyed in the Battle of the Morannon. The Orcs in Dol Guldur remained in Mirkwood until the Fall of Dol Guldur, one of the last battles of the War of the Ring.

Through the Music of the Ainur, Ilúvatar created a Vision of the World, and explained much of its nature and destiny to them - so the Ainur have much knowledge of the World, but are not omniscient. Then, Ilúvatar granted the World true being. Many of the mighty Ainur desired to descend into it and form it in readiness for the coming of the Children of Ilúvatar (Elves and Men). Fourteen of these great Ainur became the Valar, or Powers of Arda. The fifteenth, Melkor, turned aside from that path and became the first Dark Lord. The many lesser Ainur that accompanied the Valar into Arda are known as Maiar.

The Ainur are angelic beings, nonetheless they do possess gender. Those who later descended to Arda took on common male and female forms to appear to the Elves in, and have an inherent male or female gender even when formless. This is because their gender is based on their being, which is independent of any physical form they may assume.