Thomas Jefferson is considered the primary author of the Declaration of Independence, although Jefferson's draft went through a process of revision by his fellow committee members and the Second Continental Congress.
How the Declaration Came About
America's declaration of independence from the British Empire was the nation's founding moment. But it was not inevitable. Until the spring of 1776, most colonists believed that the British Empire offered its citizens freedom and provided them protection and opportunity. The mother country purchased colonists' goods, defended them from Native American Indian and European aggressors, and extended British rights and liberty to colonists. In return, colonists traded primarily with Britain, obeyed British laws and customs, and pledged their loyalty to the British crown. For most of the eighteenth century, the relationship between Britain and her American colonies was mutually beneficial. Even as late as June 1775, Thomas Jefferson said that he would "rather be in dependence on Great Britain, properly limited, than on any nation upon earth, or than on no nation."
But this favorable relationship began to face serious challenges in the wake of the Seven Years' War. In that conflict with France, Britain incurred an enormous debt and looked to its American colonies to help pay for the war. Between 1756 and 1776, Parliament issued a series of taxes on the colonies, including the Stamp Act of 1765, the Townshend Duties of 1766, and the Tea Act of 1773. Even when the taxes were relatively light, they met with stiff colonial resistance on principle, with colonists concerned that “taxation without representation” was tyranny and political control of the colonies was increasingly being exercised from London. Colonists felt that they were being treated as second-class citizens. But after initially compromising on the Stamp Act, Parliament supported increasingly oppressive measures to force colonists to obey the new laws. Eventually, tensions culminated in the shots fired between British troops and colonial militia at Lexington and Concord on April 19, 1775.
Despite the outbreak of violence, the majority of colonists wanted to remain British. Only when King George III failed to address colonists' complaints against Parliament or entertain their appeals for compromise did colonists begin to consider independence as a last resort. Encouraged by Thomas Paine’s pamphlet, “Common Sense,” more and more colonists began to consider independence in the spring of 1776. At the same time, the continuing war and rumors of a large-scale invasion of British troops and German mercenaries diminished hopes for reconciliation.
While the issue had been discussed quietly in the corridors of the Continental Congress for some time, the first formal proposal for independence was not made in the Continental Congress until June 7, 1776. It came from the Virginian Richard Henry Lee, who offered a resolution insisting that "all political connection is, and ought to be, dissolved" between Great Britain and the American colonies. But this was not a unanimous sentiment. Many delegates wanted to defer a decision on independence or avoid it outright. Despite this disagreement, Congress did nominate a drafting committee—the Committee of Five (John Adams, Benjamin Franklin, Thomas Jefferson, Robert Livingston, and Roger Sherman)—to compose a declaration of independence. Thomas Jefferson, known for his eloquent writing style and reserved manner, became the principal author.
As he sat at his desk in a Philadelphia boarding house, Jefferson drafted a "common sense" treatise in “terms so plain and firm, as to command [the] assent” of mankind. Some of his language and many of his ideas drew from well-known political works, such as George Mason's Declaration of Rights. But his ultimate goal was to express the unity of Americans—what he called an "expression of the american mind"—against the tyranny of Britain.
Jefferson submitted his "rough draught" of the Declaration on June 28. Congress eventually accepted the document, but not without debating the draft for two days and making extensive changes. Jefferson was unhappy with many of the revisions—particularly the removal of the passage on the slave trade and the insertion of language less offensive to Britons—and in later years would often provide his original draft to correspondents. Benjamin Franklin tried to reassure Jefferson by telling him the now-famous tale of a merchant whose storefront sign bore the words: "John Thompson, Hatter, makes and sells hats for ready money;" after a circle of critical friends offered their critiques, the sign merely read, "John Thompson" above a picture of a hat. 
Pressured by the news that a fleet of British troops lay off the coast of New York, Congress adopted the Lee resolution of independence on July 2nd, the day which John Adams always believed should be celebrated as American independence day, and adopted the Declaration of Independence explaining its action on July 4.
The Declaration was promptly published, and throughout July and August, it was spread by word of mouth, delivered on horseback and by ship, read aloud before troops in the Continental Army, published in newspapers from Vermont to Georgia, and dispatched to Europe. The Declaration roused support for the American Revolution and mobilized resistance against Britain at a time when the war effort was going poorly.
The Declaration provides clear and emphatic statements supporting self-government and individual rights, and it has become a model of such statements for several hundred years and around the world.
Boyd, Julian P. The Declaration of Independence: The Evolution of the Text. Issued in conjunction with an exhibit of these drafts at the Library of Congress on the two hundredth anniversary of the birth of Thomas Jefferson. Washington: Library of Congress, 1943. Reprinted 1945, 1999. Contains facsimiles of the known extant drafts of the Declaration.
Hazelton, John H. The Declaration of Independence: Its History. New York: Dodd, Mead, & Co., 1906. Reprinted 1970 by Da Capo Press. In-depth look at the creation of the Declaration of Independence. An appendix contains transcriptions of contemporary letters and annotations on the various drafts and changes to the Declaration.
National Archives. America's Founding Documents. "The Declaration of Independence." The National Archives presents a rich set of material on the Declaration, including transcripts and articles on the creation and history of the Declaration.
Milestone Documents In The National Archives. The Declaration of Independence. National Archives and Records Administration, Washington, D.C., 1992. Focuses on the history of the engrossed parchment after 1776.