Nerve growth factor (NGF) is the prototype of a family of neurotrophins (NTs), which control the development and maintenance of some vertebrate neurons in the peripheral and central nervous system. The three-dimensional structure of NGF is based on the cystine knot and β strands. It binds as a dimer to cell-surface receptors and shows its biological response. Residues involved in these interactions have been identified by chemical modification and site-directed mutagenesis. They have substantially advanced knowledge of NGF structure and function.