Zinc (Zn) is an essential heavy metal for all organisms. Zn homeostasis is maintained in mammalian cells through the activity of Zn transporters and Zn-permeable channels and through metallothionein expression levels. Zn is important in nucleic acid metabolism, cell replication, and tissue growth and repair. Zn deficiency is associated with a wide range of pathological conditions, such as impaired immunity, growth retardation, disorders in brain development, and delayed wound healing. Zn binds and affects the activity of several signaling molecules and of transcription factors that have a Zn-binding motif. However, whether Zn itself, as does calcium, acts as an intracellular signaling molecule has been a point of speculation. Recently, we and other groups have demonstrated that Zn does indeed act as an intracellular signaling molecule, converting extracellular stimuli to intracellular signals and controlling various cell functions. This chapter summarizes our current understanding of Zn signaling, especially with regard to the Zn wave and the role of Zn signaling in immune cells, and discusses how these processes contribute to allergic responses.