A review of the thermal expansion of typical shells used for investment casting is presented. Discussion of differences between types of shell systems and practical implications in the foundry is given. Data are presented for typical commercial foundry shells which are comprised of fused silica, alumino-silicate, and fused silica / alumino-silicate combinations. Data and discussion are also given for shells made of zircon-alumina which are typically used for directionally solidified and single crystal applications. The thermal expansion curve for a zircon substitute is presented. Materials for ultra-high temperature shell systems are investigated.