Treebeard is the fourth chapter of the third book in The Two Towers.
The Hobbits run as fast as they can into the forest, finally stopping for a drink of water. They come upon a sort of rock wall and climb up natural stairs, where they meet an Ent, or Treebeard, as he calls himself. Treebeard is the oldest living creature in Middle-earth, a fourteen-foot guardian of Fangorn forest. He takes the Hobbits to an Ent house and gives them drink and shelter. He knows Gandalf and asks the Hobbits for news of the outside world. When he comes to know about Saruman and how he has tamed Orcs to serve him, he is angry. He decides to gather all the Ents and march to Isengard. Treebeard tells the Hobbits that there aren't many Ents left because all the Entmaidens and Entwives have gone away and the Ents cannot find them. Due to no new Entings being born and the Ents growing old, the population is dwindling.
The next day Treebeard takes the Hobbits to a meeting of ents at Entmoot. Two dozen Ents are gathered and more come. The next day they march to Isengard. The ents are angry at Saruman, their neighbor, who cuts down their trees and burns them without reason and who also trains Orcs not to be afraid of Fangorn. Pippin looks behind and sees the whole forest moving. The trees have awakened and are marching towards Nan Curunir, the valley of Saruman.