Signaling pathways that mediate the normal functions of growth factors are commonly subverted in cancer. Oncogenes identified by a variety of approaches have been shown to funcion at critical steps in mitogenic signaling. Progression through the cell cycle requires the coordinated actions of members of two complementary classes of growth factors, and oncogenes appear to replace the actions of one set of these growth factors. Growth factors can also influence normal cell differentiation, and constitutive activation of growth-promoting pathways in cancer cells can modulate the cell phenotype as well. Paracrine actions of growth factors and cytokines may also influence the stepwise series of genetic events that lead to malignancy. New approaches for cancer therapy are being developed that intervene at various steps in growth factor signaling pathways.