A clerestory (/ˈklɪərstɔːri/ KLEER-stor-ee; lit. 'clear storey', also clearstory, clearstorey, or overstorey; from Old French: cler estor) is a high section of wall that contains windows above eye-level. Its purpose is to admit light, fresh air, or both.

Historically, a clerestory formed an upper level of a Roman basilica or of the nave of a Romanesque or Gothic church, the walls of which rise above the rooflines of the lower aisles and which are pierced with windows.

In addition to architecture, clerestories have been used in transportation vehicles such as buses and trains to provide additional lighting, ventilation, or headroom.

Clerestories appear to originate in Egyptian temples, such as the Great Hypostyle Hall, where the lighting of the hall of columns was obtained over the stone roofs of the adjoining aisles, through gaps left in the vertical slabs of stone. They appeared in Egypt at least as early as the Amarna Period. Minoan palaces in Crete such as Knossos employed lightwells in addition to clerestories.

According to the Hebrew Bible, Solomon's Temple featured clerestory windows made possible by the use of a tall, angled roof and a central ridgepole.

The clerestory was used in the Hellenistic architecture of classical antiquity. The Romans applied clerestories to basilicas of justice and to the basilica-like thermae and palaces.

Early Christian churches and some Byzantine churches, particularly in Italy, are based closely on the Roman basilica, and maintained the form of a central nave flanked by lower aisles on each side. The nave and aisles are separated by columns or piers, above which rises a wall pierced by clerestory windows.

During the Romanesque period, many churches of the basilica form were constructed all over Europe. Many of these churches have wooden roofs with clerestories below them. Some Romanesque churches have barrel-vaulted ceilings with no clerestory. The development of the groin vault and ribbed vault made possible the insertion of clerestory windows.