Alumina refractories play a key role in contemporary precision investment casting. As one of the main constituents in shell molds for high-temperature alloys, alumina is favored because of its chemical inertness and high-temperature capability. This chapter summarizes the principal physical and chemical characteristics of alumina in relation to the requirements for its use in both the slurry and stucco components of typical shell molds. Particular attention is given to factors affecting slurry stability and shell strength. Other applications for alumina in investment casting, including crucibles, alloy filters and mold insulating warp are briefly reviewed. The current trend to increasing process temperatures dictated by advances in directional solidification technology, points to expanded use of alumina refractories in the investment casting of high-temperature alloys.