Ultra-lightweight (ULW) mirror structures are defined by less than 20 % equivalent weight if compared with a solid piece of the same dimensions. The final mirror performance is primarily determined by the material properties and the structural design parameters. Quartzglass is a nearly ideal material for fabrication ULW mirrors due to its excellent weldability, its high degree of material homogeneity, its superior polishability, and its low coefficient of thermal expansion. ULW quartzglass mirrors made by HERAEUS have a sandwich structure consisting of one or two face plates and an egg-crate core structure. Back and face plates can be preshaped by a precision slumping process thus avoiding gross machining of the material to its final shape. The core structure consists of extruded square tubes flame-welded together. This construction allows for a high degree of design flexibility according to the needs of special applications. Test results obtained with a polished experimental ULW mirror of 0.5 m diameter did not reveal any structural problems.