This volume is a collection of articles written by Nobel Laureate Rita Levi-Montalcini and published from 1942 to 1995. Studies described in the first part set the stage for the discovery of a protein molecule which became known as the Nerve Growth Factor (NGF), described in detail in the second part. The NGF synthesized in minute amounts in all vertebrate tissues, plays an essential role in the differentiation and survival of several nerve cell populations in the peripheral and central nervous system. The discovery of the NGF was defined by the Nobel Foundation as a milestone in developmental neurobiology, and the author was awarded in 1986 with this prestigious award. Studies pursued in subsequent years and still in progress, have unveiled other fundamental properties of the NGF, described in the third part of this volume.