TGFbeta is a multifunctional protein which affects many proteins taking part in cell cycle regulation and, thanks to this, it influences cell growth and differentiation. TGFbeta can stimulate the proliferation of mesenchymal cells, but it can also act as a growth-inhibitory factor for epithelial, lymphatic, hematopoetic, and endothelial cells. TGFbeta may also regulate cell entry to the apoptosis pathway. TGFbeta plays an important role in angiogenesis, the stimulation of extracellular matrix synthesis, including collagen I, as well as in tissue repair and healing processes. Additionally, TGFbeta induces epithelial to mesenchymal transition in epithelial cell phenotypes. TGFbeta transmits signals through transmembrane Ser-Thr kinase receptors that directly regulate the intracellular Smad pathway. Smad proteins are a unique family of signal transduction molecules that can transmit signals directly from cell surface receptors by interacting with DNA binding partners as well as with transcriptional coactivators and corepressors. Abnormalities in TGFbeta expression are very common in many disease processes including tumors. Mutations in the genes for TGFbeta, its receptors, or intracellular signaling molecules associated with TGFbeta are also important in the pathogenesis of disease, particularly cancer.