Cytokines and growth factors are signaling molecules that bind to specific receptors on the surface of their target cells and induce signaling pathways, which then regulate an array of biological processes. Numerous diverse molecules are categorized into different families of cytokines and/or growth factors. Members of these families regulate processes such as cell proliferation, activation, differentiation, and migration. In addition, growth factors control processes required for development including tissue organization, morphogenesis, and angiogenesis. In adult organisms, growth factors are also involved in regulating metabolism, wound healing, and maintaining tissue homeostasis. Constitutive activation of growth factor signaling is frequently associated with cellular transformation and tumorigenesis.