Growth factors are extracellular signaling molecules that act in an autocrine and paracrine fashion to regulate growth, proliferation, differentiation, and survival of cells. Dysregulation of the growth factor networks is intimately related to the molecular pathogenesis of neoplastic and paraneoplastic disease. Increasing knowledge of the molecular mechanisms underlying growth factors and their actions on cell cycling, cell division, and cell death is shedding light on new therapeutic avenues for molecular targeting of tumors. Epidermal growth factor and vascular endothelial growth factor both offer examples of how growth factor biology and its relationship to cancer can be harnessed to create effective clinical therapeutic tools such as monoclonal antibodies. This approach heralds a future in which rational molecular oncological therapy may increasingly become the norm.