The Great Chicago Fire was a conflagration that burned from Sunday, October 8, to early Tuesday, October 10, 1871, killing hundreds and destroying about 3.3 square miles (9 km2) in Chicago, Illinois. Though the fire was one of the largest U.S. disasters of the 19th century, Chicago was rebuilt and continued to grow as one of the most populous and economically important American cities. The fire began the same day as several other fires destroyed towns and forests in Wisconsin and Michigan.

The fire started at about 21:00 on Sunday, October 8, in or around a small barn that bordered the alley behind 137 DeKoven Street. The traditional account of the origin of the fire is that it was started by a cow kicking over a lantern in the barn owned by Patrick and Catherine O'Leary. In 1893, Michael Ahern, the Chicago Republican reporter who wrote the O'Leary account, admitted he had made it up as colorful copy. The barn was the first building to be consumed by the fire, but the official report could not determine the exact cause.

The fire's spread was aided by the city's use of wood as the predominant building material, a drought prior to the fire, and strong winds from the southwest that carried flying embers toward the heart of the city. More than 2/3 of the structures in Chicago at the time of the fire were made entirely of wood. Most houses and buildings were topped with highly flammable tar or shingle roofs. Most Chicago architects modeled wooden building exteriors after another material using ornate, decorative carvings. All the city's sidewalks and roads were also made completely out of wood.