When the Atlas of the Lewis and Clark Expedition appeared in 1983 critics hailed it as a publishing landmark in western history. Fully living up to the promise of the first volume were the second volume, which began the actual journals and brought the expedition through its first year to August 1804, and the third volume, which brought the explorers through a winter at Fort Mandan, present North Dakota, and to April 1805. This eagerly awaited fourth volume begins on April 7, 1805, when Meriwether Lewis and William Clark and their permanent party set out from Fort Mandan, traveling up-river along the banks of the Missouri. For the first time they entered country never explored by whites. With the help of the Shoshone Indian woman Sacagawea, they hoped to make friendly contact with her people, then cross the Rocky Mountains and eventually reach the Pacific. They were to spend the rest of the spring and the early summer toiling up the Missouri, or around its perilous falls. Along the way, they encountered grizzly bears, cataloged new species of plants and animals, and mapped rivers and streams. Sacagawea recognized landmarks; meeting her people became the next great concern of the expedition when they reached the three forks of the Missouri in late July. Superseding the last edition, published early in this century, the current edition contains new materials discovered since then. It expands and updates the annotation to take account of the most recent scholarship on the many subject touched on by the journals.