For many years Schott has been supplying the Zerodur glass ceramic, a very low expansion material, for a variety of applications, in particular, mirror substrates. With the requirements shifting towards larger diameters, a decrease in the weight of the mirror substrate becomes increasingly important. This goal can be achieved by applying a supporting rib structure to a thin face sheet thus providing the structural rigidity or by actually actively supporting a thin meniscus shaped blank. Considerable progress has been made in the forming of Zerodur, in particular, by applying the fusion, slumping and spincasting techniques. Several samples have been fused with diameters up to .5 m and aerial densities around 60 kg/m2. The slumping technique has been demonstrated for diameters up to 1 m with a thickness range between 4 and 100 mm. The spincasting technique has been successfully developed and demonstrated by the manufacture of several mirror blanks up to a diameter of 4.1 m at thicknesses of down to 57 mm. The production of spincast Zerodur mirror substrates is now deemed feasible for diameters larger than 8 m. The parameters for the selection between the slumping and spincasting processes are geometry, flexibility and economy.