Thomas Jefferson (April 13, 1743 - July 4, 1826) was the third President of the United States (1801-1809), the principal author of the Declaration of Independence (1776), and one of the most influential Founding Fathers for his promotion of the ideals of republicanism in the United States. Major events during his presidency include the Louisiana Purchase (1803) and the Lewis and Clark Expedition (1804-1806).