A curtain wall system is an outer covering of a building in which the outer walls are non-structural, utilized to keep the weather out and the occupants in. Since the curtain wall is non-structural, it can be made of lightweight materials, thereby reducing construction costs. When glass is used as the curtain wall, an advantage is that natural light can penetrate deeper within the building. The curtain wall façade does not carry any structural load from the building other than its own dead load weight. The wall transfers lateral wind loads that are incident upon it to the main building structure through connections at floors or columns of the building. A curtain wall is designed to resist air and water infiltration, absorb sway induced by wind and seismic forces acting on the building, withstand wind loads, and support its own dead load weight forces.

Curtain wall systems are typically designed with extruded aluminum framing members, although the first curtain walls were made with steel frames. The aluminum frame is typically infilled with glass, which provides an architecturally pleasing building, as well as benefits such as daylighting. However, the effects of light on visual comfort as well as solar heat gain in a building are more difficult to control when using large amounts of glass infill. Other common infills include: stone veneer, metal panels, louvres, and operable windows or vents.