The semaphorins were initially described as axon guidance factors, but have recently been implicated in a variety of physiological and developmental functions, including regulation of immune response, angiogenesis, and migration of neural crest cells. The semaphorin family contains more than 30 genes divided into seven subfamilies, all of which are characterized by the presence of a sema domain. The semaphorins transduce their signals by binding to one of the nine receptors belonging to the plexin family, or, in the case of the class 3 semaphorins, by binding to one of the two neuropilin receptors. Additional receptors, which form complexes with these primary semaphorin receptors, are also frequently involved in semaphorin signaling. Recent evidence suggests that some semaphorins can act as antiangiogenic and/or antitumorigenic agents whereas other semaphorins promote tumor progression and/or angiogenesis. Furthermore, loss of endogenous inhibitory semaphorin expression or function on one hand, and overexpression of protumorigenic semaphorins on the other hand, is associated with the progression of some tumor types.