Acidic fibroblast growth factor (FGF-1) and basic fibroblast growth factor (FGF-2) are ubiquitous cytokines found in many tissues. They have effects on multiple cell types derived from mesoderm and neuroectoderm, including endothelial cells. FGF proteins are small peptides of 155 to 268 amino acid residues. The degree of sequence identity between different family members is 30-60 % in a "central domain" of approx. 120 amino acids. This domain confers to FGFs a common tertiary structure and the ability to bind to heparin. Secreted FGFs signal to target cells by binding and activating cell-surface tyrosine kinase FGF receptors (FGFRs; 6, 7). The function of FGFs and FGFRs during embryonic development and adult physiology has been addressed by gain- and loss-of-function experiments in several animal model organisms. These studies have shown that FGFs act as key regulators of developmental events.