A curtain wall is an outer covering of a building in which the outer walls are non-structural, designed only to keep the weather out and the people in. Because the curtain wall façade carries no structural load beyond its own dead load weight, it can be made of lightweight materials. The wall transfers lateral wind loads upon it to the main building structure through connections at floors or columns of the building.

Curtain walls may be designed as "systems" integrating frame, wall panel, and weatherproofing materials. Steel frames have largely given way to aluminum extrusions. Glass is typically used for infill because it can reduce construction costs, provide an architecturally pleasing look, and allow natural light to penetrate deeper within the building. But glass also makes the effects of light on visual comfort and solar heat gain in a building more difficult to control. Other common infills include stone veneer, metal panels, louvres, and operable windows or vents.

Unlike storefront systems, curtain wall systems are designed to span multiple floors, taking into consideration building sway and movement and design requirements such as thermal expansion and contraction; seismic requirements; water diversion; and thermal efficiency for cost-effective heating, cooling, and interior lighting.