Valley Forge was the winter encampment of the Continental Army, under the command of George Washington, during the American Revolutionary War. The Valley Forge encampment lasted six months, from December 19, 1777, to June 19, 1778.

Three months prior to the encampment at Valley Forge, in September 1777, the Second Continental Congress was forced to flee the revolutionary capital of Philadelphia ahead of an imminent British attack on the city following Washington's defeat in the Battle of Brandywine, a key battle during the British Army's Philadelphia campaign. Unable to defend Philadelphia, Washington led his 12,000-man army into winter quarters at Valley Forge, located approximately 18 miles (29 km) northwest of Philadelphia.

At Valley Forge, the Continental Army struggled to manage a disastrous supply crisis while simultaneously retraining and reorganizing their units in an effort to mount successful counterattacks against the British. During the encampment at Valley Forge, an estimated 1,700 to 2,000 soldiers died from disease, possibly exacerbated by malnutrition and cold, wet weather. In 1976, in recognition of the enormous historical significance of Valley Forge in American history, Valley Forge National Historical Park was established and named a national historic site, which protects and preserves nearly 3,600 acres (1,500 ha) of the original Valley Forge encampment site. The park is a popular tourist destination, drawing nearly 2 million visitors each year.

In 1777, Valley Forge consisted of a small proto-industrial community located at the confluence of Valley Creek and the Schuylkill River. In 1742, Quaker industrialists established the Mount Joy Iron Forge in Valley Forge. Capital improvements made by John Potts and his family over the following decades permitted the small community to expand its ironworks, establish mills, and construct new dwellings for the village's residents. Valley Forge was surrounded by rich farmland, where mainly Welsh Quaker farmers grew wheat, rye, hay, Indian corn, and other crops, and raised livestock, including cattle, sheep, pigs, and barnyard fowl. Settlers of German and Swedish descent also lived nearby.

In the summer of 1777, the Continental Army's quartermaster general, Thomas Mifflin, decided to station a portion of the Continental Army's supplies in buildings in the village because the area had a variety of structures and was a secluded location surrounded by two hills. Fearing such a concentration of military supplies would become a target for British raids, the forge's ironmaster, William Dewees Jr., expressed concerns about the army's proposal. Mifflin heeded Dewees' concerns but established a magazine at Valley Forge anyway.

After the British landed at the Head of Elk in present-day Elkton, Maryland, on August 25, 1777, the British Army maneuvered out of the Chesapeake basin and headed north as part of the Philadelphia campaign. Following the Battle of Brandywine on September 11, 1777, and the abortive Battle of the Clouds five days later, on September 16, several hundred soldiers under General Wilhelm von Knyphausen raided the supply magazine at Valley Forge on September 18.

Despite the best efforts of Lieutenant Colonel Alexander Hamilton and Captain Henry "Light Horse Harry" Lee, the two Continental Army officers tasked with evacuating the supplies at Valley Forge, British soldiers captured supplies, destroyed others, and burned down the buildings used as supplies stores, including the forges and other buildings.

Political, strategic, and environmental factors all influenced the Continental Army's decision to establish their encampment near Valley Forge, in the winter of 1777–1778. The site was selected after George Washington conferred with his officers to select a strategic location that would prove most advantageous to the Continental Army.