III-V semiconductors have attracted considerable attention due to their applications in the fabrication of electronic and optoelectronic devices as light emitting diodes and solar cells. The electrical properties of these semiconductors can also be tuned by adding impurity atoms. Because of their wide application in various devices, the search for new semiconductor materials and the improvement of existing materials is an important field of study. Doping with impurities is a common method of modifying and diversifying the properties of physical and chemical semiconductors. This book covers all known information about phase relations in quaternary systems based on III-V semiconductors.