Class 3 semaphorins (SEMA3) were first identified as glycoproteins that negatively mediate neuronal guidance by binding to neuropilin and repelling neurons away from the source of SEMA3. However, studies have shown that SEMA3s are also secreted by other cell types, including tumor cells, where they play an inhibitory role in tumor growth and angiogenesis (specifically SEMA3B and SEMA3F). SEMA3s primarily inhibit the cell motility and migration of tumor and endothelial cells by inducing collapse of the actin cytoskeleton via neuropilins and plexins. Besides binding to SEMA3s, neuropilin also binds the protumorigenic and proangiogenic ligand vascular endothelial growth factor (VEGF). Although some studies attribute the antitumorigenic and antiangiogenic properties of SEMA3s to competition between SEMA3s and VEGF for binding to neuropilin receptors, several others have shown that SEMA3s display growth-inhibitory activity independent of competition with VEGF. A better understanding of these molecular interactions and the role and signaling of SEMA3s in tumor biology will help determine whether SEMA3s represent potential therapeutic agents. Herein, we briefly review (a) the role of SEMA3s in mediating tumor growth, (b) the SEMA3 receptors neuropilins and plexins, and (c) the potential competition between SEMA3s and VEGF family members for neuropilin binding.