More than 50 PTK receptors are known to be involved in regulation of cell growth, differentiation, chemotaxis and actin reorganization. PTK receptors can be classified into subfamilies according to their structural features. PTK receptors are activated by ligand induced homo- or heterodimerization, which leads to receptor autophosphorylation on tyrosine residues. In certain receptors, the autophosphorylation regulates the catalytic activity of the kinase. Moreover, autophosphorylated tyrosine residues bind signal transduction molecules with SH2 or PTP domains. Such molecules are activated by the actual binding to the receptors or by phosphorylation on tyrosine residues by the receptor kinase. There are also examples of constitutively active signal transduction molecules that are translocated to act at the cell membrane by binding to autophosphorylated PTK receptors. In this way, specific intracellular signal transduction pathways are initiated. After ligand binding and activation, PTK receptors are internalized and deactivated by dephosphorylation as well as by degradation in the cytoplasm or in the lysosomes.