Cellular signaling centers are a cluster of nonproliferating cells that secrete molecular signals that regulate cell behaviors and the fate(s) of surrounding cells. This chapter focuses on cellular signaling centers identified in vertebrates. However, important concepts leading to the idea of cellular signaling centers began in the 1970s in Drosophila. A cellular signaling center is defined as a small, focused number of non-dividing cells that secrete signals to direct the behaviors of surrounding cells. These cell clusters are temporary; the cells die when the maintenance factors diminish and eventually disappear (Jernvall et al.). As more details of developmental signaling networks have become available, researchers have described more signaling centers in the developing embryo, revealing that these temporary signaling centers are a common mechanism for directing morphogenesis in the embryo and for enabling evolutionary change. As more signaling centers are identified, it has become clear that signaling centers are prevalent in embryogenesis, used by many, if not most, organ-systems.