This thesis has been made in order to research the effect of Z-phase in high temperature steels. The thesis contains a general description of high temperature steels. The emphasis will be made on a thermodynamical description of Z-phase and published material containing information about the Z-phase, which has been gathered. An experiment was made in order to check the stability of Z-phase in different modern steels, but it had to be cut short because of equipment problems. Further research on the thermodynamical description will be carried out in the future, and work on a model in Thermo-calc will be commenced.