A siege is a prolonged military blockade and assault of a city or fortress with the intent of conquering by force or attrition. A siege occurs when an attacker encounters a city or fortress that refuses to surrender and cannot be easily taken by a frontal assault. Sieges usually involve surrounding the target and blocking the provision of supplies, typically coupled with Siege engines, artillery bombardment or sapping (also known as mining) to reduce fortifications.

Sieges probably predate the development of cities as large population centers. Ancient cities in the Middle East show archeological evidence of having had fortified city walls. During the Renaissance and the Early Modern period, siege warfare dominated the conduct of war in Europe. Leonardo da Vinci gained as much of his renown from the design of fortifications as from his artwork.

Medieval campaigns were generally designed around a succession of sieges. In the Napoleonic era, increasing use of ever more powerful cannon reduced the value of fortifications. In modern times, trenches replaced walls, and bunkers replaced castles. In the 20th century, the significance of the classical siege declined. With the advent of mobile warfare, one single fortified stronghold is no longer as decisive as it once was. While sieges do still occur, they are not as common as they once were due to changes in modes of battle, principally the ease by which huge volumes of destructive power can be directed onto a static target.